JP6222041B2 - Ultra-thick steel plate with excellent HIC resistance and manufacturing method thereof - Google Patents

Description

  The present invention requires an ultra-thick steel plate used in a wet hydrogen sulfide environment typified by pressure vessels such as petroleum refining planners and process piping, in particular, excellent HIC (Hydrogen Induced Cracking) performance. The present invention relates to a method for manufacturing an extremely thick steel plate used for a member. In addition, the extremely thick steel plate in the present invention is a steel plate having a thickness of 50 mm or more.

  The amount of crude oil mining is increasing year by year on the back of rising global energy demand, and high-quality crude oil like the conventional one is gradually depleted, and it is necessary to use low-grade crude oil with high hydrogen sulfide concentration. It has been. For this reason, even in pressure vessels and process piping used for petroleum refining plan droughts, it is not suitable for wet hydrogen sulfide environments where hydrogen induced cracking (HIC) or sulfide stress corrosion cracking (SSC) does not occur. Steel plates having resistance, i.e., sour resistance (HIC resistance or SSC resistance) are often used. HIC is known to occur even in a relatively low strength steel sheet, which is a particular problem.

  Studies to ensure the HIC resistance performance of steel sheets are actively conducted mainly in the field of line pipes. For example, 1) Reduction of elements concentrated in the central segregation part of continuous cast slabs such as Mn and P Reduction of center segregation part by optimization of casting conditions, 2) Reduction of S and O, and suppression of generation of MnS and addition of Ca cluster caused by addition of Ca by addition of optimum amount of Ca, 3) Accelerated cooling and heat treatment in TMCP By homogenizing the microstructure by applying quenching in the process, the distribution of C to the central segregation part due to the formation of ferrite and the suppression of the complex structure that becomes the HIC propagation path are suppressed. 4) MA generated with high-strength material Suppression of formation of hard second phase such as (Martensite-Austenite constituent, island martensite), decomposition by reheating Such as has been carried out.

  However, all of these findings are findings relating to steel plates and steel pipes for line pipes having a plate thickness of less than 50 mm. Therefore, even if the above-described method is applied to an extremely thick material having a plate thickness of 50 mm or more, the target performance is not necessarily obtained. In particular, there is a problem that as the plate thickness increases, an uncompressed portion called zaku or porosity remains after rolling and becomes the starting point of HIC. Further, since it is necessary to add a large amount of alloy elements to obtain a desired strength, there is a problem that the center segregation portion is further hardened and HIC is likely to occur.

  For such a problem, Patent Document 1 uses a combination of taking a large total rolling reduction ratio from split rolling to finish rolling using an ingot slab and reducing the amount of hydrogen contained in the steel sheet, A method for manufacturing a very thick steel plate that satisfies both HIC resistance and internal quality is disclosed. Patent Document 2 discloses a method for producing a thick steel plate that secures HIC resistance by using only a portion in which a chemical component such as S becomes a desired value by unidirectionally solidifying from the lower part of the mold to the upper part by an ingot forming method. Has been. In Patent Document 3, when producing an extremely thick steel plate of 100 mm or more from a continuous cast slab, the continuous cast slab is held at a high temperature for 20 to 40 hours, and then subjected to strong pressure forging to diffuse the alloy elements in the central segregation part. In addition, a method for producing an extremely thick steel sheet that achieves both excellent HIC resistance and internal quality by performing pulverization, reheating, and hot rolling is disclosed. In Patent Documents 4 to 7, the alloy element of the central segregation part is obtained by performing hot rolling under a large pressure after holding for a predetermined time or more at a predetermined heating temperature obtained from a chemical component prior to normal hot rolling. A method of manufacturing a thick steel sheet that achieves both excellent HIC resistance and internal quality by performing diffusion of the above, reheating, and applying optimal TMCP conditions is disclosed. In Patent Document 8, after reducing the central segregation part by the same method as Patent Documents 4 to 7, the steel sheet is adjusted by adjusting the properties of the steel sheet by normalization, thereby obtaining a thick steel sheet that achieves both excellent HIC resistance and internal quality. A manufacturing method is disclosed.

JP-A-4-329826 Japanese Patent Laid-Open No. 62-176601 JP 2001-89812 A Japanese Patent Laid-Open No. 2-173208 Japanese Patent Laid-Open No. 4-263017 Japanese Patent Laid-Open No. 5-125441 JP-A-5-295435 JP-A-4-143217

  However, all of the methods disclosed in Patent Documents 1 and 2 use ingot slabs, and the productivity is extremely poor. On the other hand, in the method disclosed in Patent Document 3, since the continuous casting slab is heated and held for a long time and then forged under strong pressure, the center segregation degree and the internal quality are good and excellent HIC resistance can be ensured. However, productivity is poor because the heating and holding time is too long. In addition, in the methods disclosed in Patent Documents 4 to 8, when producing an extremely thick steel plate using a continuous cast slab, sufficient internal quality cannot be ensured, and the HIC is based on abnormal internal quality and zaku or porosity. May not be suppressed.

  As described above, in the conventional technology, it is possible to manufacture a very thick steel plate excellent in HIC resistance without causing an increase in cost, a decrease in productivity, a deterioration in internal quality, and a deterioration in HIC resistance performance resulting therefrom. It was difficult. Accordingly, an object of the present invention is to provide a low-cost, high-productivity manufacturing method that ensures excellent internal quality and anti-HIC performance even with an extremely thick steel plate having a thickness of 50 mm or more.

  In order to solve the above problems, the present inventors have earnestly studied forging conditions that affect the internal quality of the slab and chemical components that affect the center segregation hardness and inclusions, forging conditions, and rolling conditions. The following findings were obtained. First, as a result of examining forging conditions that affect the internal quality of continuous cast slabs, the HIC starting point after hot rolling is ensured by ensuring a reduction rate per pass of 5% or more and a total reduction rate of 10% or more. It was found that a rolled material excellent in internal quality can be obtained by crimping such zaku and porosity.

  Next, the reduction of MnS-based and NbCN-based inclusions, which are the origin of HIC at the central segregation part, was examined. Both MnS and NbCN in the central segregation part are crystallized elements concentrated in the final solidification part of the casting, and up to the melting temperature of inclusions theoretically determined from the components and solubility products of the normal bulk part Even when heated and held, the inclusions do not dissolve. On the other hand, it was found that the solid solution temperature of inclusions can be obtained more accurately by considering the concentration of the alloy element in the central segregation part. Based on the above concept, the inventors have come up with SMNS (Equation (3) described later) and SNBCN (Equation (4) described later). When both become 0 or more, most of MnS and NbCN in the central segregation part can be dissolved, and it can be rendered harmless without starting the HIC. NbCN, when Ti is added to the steel, is complexed with TiN and hardly dissolves. However, by setting SNBCN to 0 or more, the cluster diameter of NbCN can be reduced, and excellent HIC resistance can be ensured by suppressing the center segregation portion hardness described below to a certain level or less.

Moreover, the inclusion composition for suppressing generation | occurrence | production of HIC resulting from the cluster of an Al-Ca type oxide was examined. As a result, it is possible to lower the melting point of the inclusion by controlling the Al ₂ O ₃ : CaO composition ratio of the Al—Ca-based oxide to about 1: 1, and the Ca—Al-based inclusion in the molten steel can be lowered. It was found that aggregation can be suppressed and clustering can be suppressed.

  Next, the diffusion behavior during forging heating and rolling heating of the element concentrated in the center segregation portion was investigated. As a result, it was found that for Mn and P, in heating at 1200 ° C. or higher, which is usually performed by forging heating, the concentration at the center segregation part can be reduced as the holding time is increased. Further, since C is an element that diffuses very easily, it was found that the microstructure became a two-phase structure during air cooling after forging, and C diffused by forging again concentrated in the central segregation part. Furthermore, it is found that C diffuses easily even when heating at 1200 ° C. or less, which is usually performed in rolling heating, and that concentration at the center segregation part can be reduced as the temperature during heating and holding, and the forging reduction ratio increase. It was. On the other hand, it was also found that elements such as Cu, Ni, Cr, Mo, and V hardly diffuse by these heating. Based on the above results, Pcm was proposed as an index of center segregation hardness. Pcm is an index that quantifies the influence of alloy elements, forging conditions, and rolling heating conditions on the center segregation hardness (formula (2) described later). In the case of a steel sheet manufactured by controlling the microstructure to bainite and reheating (quenching), or by reheating and tempering, the hardness of the central segregation part is lowered by setting this value to 0.280 or less. It was found that excellent performance can be obtained by the HIC test of the NACE TM0284-A solution even when there are fine inclusions having a size of less than 200 μm and their accumulation bands. By using Pcm, it becomes possible to select a more rational slab component design and steel plate manufacturing conditions than in the past.

The present invention has been made by further studying the above findings and is as follows.
[1] In mass%, component composition is C: 0.030 to 0.200%, Si: 0.50% or less, Mn: 0.60 to 1.60%, P: 0.020% or less, S : 0.0015% or less, Al: 0.060% or less, Ca: 0.0010 to 0.0040%, N: 0.0050% or less, O: 0.0030% or less, and Cu: 0 50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.100% or less, Ti: 0.020 % Or less, B: 0.0030% or less of one or more, Pcm represented by the formula (1) is 0.280 or less, the balance Fe and unavoidable impurities, The microstructure at 8t (t is the plate thickness) to 7 / 8t position has an old auto with an aspect ratio of 1.5 or less. Bainite generated from tenite grains accounts for 90% or more of the total microstructure, the Vickers hardness of the central segregation part is 300 or less, and voids, MnS inclusions, Nb or Ti or both present in the central segregation part are included. Inclusions composed of compounds containing, inclusion clusters composed of compounds containing Al or Ca, or both, each have a major axis of less than 200 μm, and more than 30% of the number of oxides containing Al and Ca in steel An extra-thick steel plate with excellent HIC resistance having a thickness of 50 mm or more, which is an oxide having a molar ratio of Al ₂ O ₃ /CaO=0.7 to 1.3.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
However, in said formula (1), each element symbol is content (mass%), and is set to 0 when not containing.
[2] A continuous cast slab having the component composition described in [1] obtained by continuously casting molten steel is held at a heating temperature Ta of 1000 to 1350 ° C. for 300 min or more, and then per pass. The rolling reduction is 5% or more, the total rolling reduction R is 10% or more, the PCCT represented by the formula (2) is 1.05 or less, the SMNS represented by the formula (3) is 0 or more, and the SNBCN represented by the formula (4) Is air-cooled after hot forging under the condition that becomes 0 or more, and then reheated at a heating temperature T _{b of} 880 to 1300 ° C. and a holding time t _b , hot-rolled, then air-cooled, Further, after reheating to a temperature of 880 ° C. to 1100 ° C., the center of the plate thickness is cooled at a cooling rate of 4 ° C./s or more from a temperature range of Ar ₃ points or more, and a HIC resistance performance of a plate thickness of 50 mm or more For producing ultra-thick steel plates with excellent resistance.
PCCT = CP + MnP / 6 + 0.116Cu + 0.113Ni + 0.236Cr + 0.390Mo + 0.348V + 2PP (2)
SMNS = T _a + 273-5560 / (0.72-log [1.2Mn] [S]) (3)
SNBCN = T _a + 273-6770 / (2.26-log [5Nb] [C + 12N / 14]) (4)
However, CP, MnP and PP are values calculated by the following formulas (2-1) to (2-3). In the above formulas (2) to (4), Cu, Ni, Cr, Mo, V, Mn, Nb, C, and N are the contents (mass%) of each element, and 0 when not contained. To do. Ta is the heating temperature during hot forging.
CP = C + (0.77−C) erf (795 · R / 400000000 / (((0.000023exp (−17800 / (T _b +273))) 60t _b ) ^0.5 )) (2-1) )
MnP = Mn + 1.702 Mn · erf (795 / 4000,000 / (((0.00004exp (−31511 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-3)
However, in the above formulas (2-1) to (2-3), C, Mn, and P are set to 0 when not contained in the content (mass%) of each element. Ta is heating temperature at the time of hot forging (℃), t _a heating holding time at the time of hot forging (min), R is the total reduction ratio during hot forging (%), T _b is the time of hot rolling the heating temperature (℃), _{t b} is the heating holding time at the time of hot rolling (min). Note that erf is an error function.
[3] The method for producing an extra-thick steel plate having excellent HIC resistance with a thickness of 50 mm or more according to [2], wherein after cooling, air cooling is performed, and a tempering heat treatment is performed in a temperature range of 480 to 720 ° C.
[4] Insol. Analyzing Al and adding Ca so that Al ₂ O ₃ / CaO in molten steel is 0.7 to 1.3 in molar ratio based on the analysis result [2] or The method for producing an extra-thick steel plate having excellent HIC resistance performance with a thickness of 50 mm or more according to [3].

  According to the present invention, it is possible to manufacture an extremely thick steel plate excellent in internal quality and anti-HIC performance with low cost and high productivity, which is extremely effective industrially.

  Hereinafter, the present invention will be specifically described.

1. Component composition Below, the reason for limitation of the component composition of the steel material which concerns on this invention is demonstrated. In addition,% of the unit which shows a component composition means the mass% altogether.

C: 0.030 to 0.200%
C is inexpensive and contributes to high strength. On the other hand, it is an element that deteriorates the degree of central segregation and should be reduced from the viewpoint of securing HIC resistance. However, if C is lower than 0.030%, the hardenability becomes too low to obtain a desired strength, and the microstructure cannot be controlled to a uniform bainite structure, so the lower limit is made 0.030%. . On the other hand, if it exceeds 0.200%, the degree of segregation deteriorates and the HIC resistance cannot be secured, so the upper limit is made 0.200%. More preferably, it is 0.030 to 0.180%.

Si: 0.50% or less Si is a deoxidizing element and is unavoidably contained in the slab. Further, it is an element that can be increased in strength without taking into consideration the enhancement of the hardenability of the center segregation part. If it is 0.50% or less, the weldability and weld heat affected zone toughness are not deteriorated so much, so the upper limit is made 0.50%. More preferably, it is 0.40% or less.

Mn: 0.60 to 1.60%
Mn is an element that enhances hardenability. However, it tends to concentrate in the center segregation part and deteriorates the HIC resistance. If Mn exceeds 1.60%, the occurrence of HIC cannot be suppressed even if other conditions such as chemical components and forging conditions are adjusted, so the upper limit is made 1.60%. Moreover, since the intensity | strength of a very thick steel plate cannot be ensured when Mn is less than 0.60%, a minimum is made into 0.60%. More preferably, it is 0.80-1.50%, More preferably, it is 0.90-1.40%.

P: 0.020% or less P is an element that is inevitably contained and also enhances hardenability. However, it is very easy to concentrate in the center segregation part, and has a very bad influence on the HIC resistance. Therefore, although it is preferable to strengthen the de-P treatment in the steelmaking process and reduce it as much as possible, it is very costly to reduce P. Further, in the present invention, the concentration of P at the center segregation portion during forging heating can be alleviated. If P is 0.020% or less, the HIC resistance can be ensured by the effect, so the upper limit is set to 0. 020%. More preferably, it is 0.015% or less, More preferably, it is 0.010% or less.

S: 0.0015% or less S is an element inevitably included. However, since MnS is formed and becomes the starting point of HIC, it affects the anti-HIC performance. Therefore, it is better to reduce as much as possible. However, strengthening desulfurization leads to an increase in cost. For this reason, 0.0015% which is the upper limit which can anticipate the production | generation suppression effect of MnS by Ca addition is accept | permitted. More preferably, it is 0.0010% or less, More preferably, it is 0.0008% or less.

Al: 0.060% or less Al is a deoxidizing element and is unavoidably contained in the slab. If Al exceeds 0.060%, HIC due to Al ₂ O ₃ clusters is generated, so the upper limit is made 0.060%. Preferably, it is 0.050% or less, More preferably, it is 0.040% or less.

Ca: 0.0010 to 0.0040%
Ca can suppress the production | generation of the extended MnS by couple | bonding with S ahead of Mn at the time of casting, and producing | generating CaOS and CaS. The effect does not appear unless 0.0010% or more is added, so the lower limit is made 0.0010%. Further, if added over 0.0040%, CaO and CaOS are excessively generated, and a cluster is formed to serve as an HIC starting point, so that the HIC resistance is deteriorated, so the upper limit is made 0.0040%.

N: 0.0050% or less N is an element inevitably contained in the steel, and combines with Ti to produce TiN. When Ti is added, if N exceeds 0.0050%, TiN crystallized may become the starting point of HIC. On the other hand, when Ti is not added, there are many solid solution N, and the surface crack of a slab generate | occur | produces. For this reason, the upper limit is made 0.0050%. More preferably, it is 0.0045% or less.

O: 0.0030% or less O is an element inevitably contained in steel, and most of it exists as an oxide. If the amount of O exceeds 0.0030%, the amount of inclusions is large and internal quality and HIC resistance cannot be ensured, so the upper limit is made 0.0030%.

Pcm: 0.280 or less Pcm is an index for quantifying the hardenability of the steel material. If this value exceeds 0.280, the hardenability becomes too high to ensure the HIC resistance, so the upper limit is set to 0.280. More preferably, it is 0.250 or less. Pcm is represented by the following formula (1).
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
However, in said formula (1), each element symbol is content (mass%), and is set to 0 when not containing.

  In the present invention, in addition to the above component composition, in order to obtain strength and toughness, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, One or more of Nb: 0.050% or less, V: 0.100% or less, Ti: 0.020% or less, and B: 0.0030% or less are contained.

Cu: 0.50% or less Cu increases the strength of a steel sheet by solid solution strengthening. On the other hand, when Cu is added excessively, weldability and toughness deteriorate, and the cost increases. For this reason, when it contains, an upper limit shall be 0.50%.

Ni: 1.00% or less Ni increases the strength of the steel sheet by solid solution strengthening and further increases the toughness of the matrix structure. On the other hand, if Ni is added excessively, the weldability deteriorates and the cost increases. For this reason, when it contains, an upper limit shall be 1.00%.

Cr: 0.50% or less Cr increases the hardenability and increases the strength of the steel sheet. On the other hand, when Cr is added excessively, weldability and toughness deteriorate, and the cost increases. For this reason, when it contains, an upper limit shall be 0.50%.

Mo: 0.50% or less Mo increases the hardenability and increases the strength of the steel sheet. On the other hand, when Mo is added excessively, weldability and toughness deteriorate, and the cost increases. For this reason, when it contains, an upper limit shall be 0.50%.

Nb: 0.050% or less Nb expands the non-recrystallization region temperature during controlled rolling, and contributes to refinement of the structure during rolling. On the other hand, if added over 0.050%, precipitation embrittlement is caused. Furthermore, the presence of coarse NbCN generated in the center segregation part deteriorates the HIC resistance. For this reason, when it contains, an upper limit shall be 0.050%. More preferably, it is 0.040% or less.

V: 0.100% or less V increases the strength of the steel sheet mainly by precipitation strengthening. On the other hand, when V is added excessively, toughness is remarkably impaired. For this reason, when it contains, an upper limit shall be 0.100%.

Ti: 0.020% or less Ti refines the structure by forming TiN. In particular, it reduces the coarse grain region width when welding and significantly improves the weld heat affected zone toughness. On the other hand, when Ti is added excessively, coarse TiN is crystallized during solidification and the HIC resistance is deteriorated. For this reason, when it contains, an upper limit shall be 0.020%.

B: 0.0030% or less B is an element that increases hardenability, and is an element that is very effective for increasing the strength. On the other hand, if added over 0.0030%, the hardenability becomes too high, the hardness of the steel sheet surface layer and the weld heat affected zone is increased, and the SSC resistance is deteriorated. For this reason, when it contains, an upper limit shall be 0.0030%.

  The balance other than the above components is Fe and inevitable impurities.

2. Microstructure of extra-thick steel plate Microstructure at 1 / 8t (t is the plate thickness) to 7 / 8t position from the surface layer: Bainite formed from prior austenite grains with an aspect ratio of 1.5 or less is 90% or more resistant to the total microstructure From the viewpoint of securing HIC performance, it is desirable that the microstructure is uniform. In addition, from the viewpoint of securing strength, it is necessary to use mainly bainite rather than ferrite. In the case of an extremely thick steel plate, the vicinity of the surface layer may undergo ferrite transformation and a uniform bainite structure may not be obtained. However, since there is no segregation or inclusion accumulation zone in the vicinity of the surface layer, there is no problem in securing HIC resistance. Therefore, in the present invention, the bainite main body is assumed at the position 1 / 8t to 7 / 8t from the surface layer. As for the bainite fraction, if the area ratio is 90% or more, the HIC does not propagate significantly due to the non-uniformity of the microstructure. For this reason, a minimum is made into 90%. Moreover, since the HIC performance improves as the aspect ratio of the prior austenite grains is closer to 1 in the bainite structure, the aspect ratio of the prior austenite grains is 1.5 or less in the present invention. In the present invention, the structure other than bainite means ferrite, martensite and pearlite, and minute cementite and MA present in bainite are regarded as a part of bainite.

Vickers hardness of the center segregation part: 300 or less In the center segregation part, inclusions serving as starting points of HIC such as MnS, NbCN, TiN, Al ₂ O ₃ , and CaOS are generated. In the present invention, MnS and NbCN are dissolved by heating during forging. However, since TiN, Al ₂ O ₃ , and CaOS cannot be dissolved, it can be a starting point of HIC generation. In order to suppress the generation and propagation of HIC even when these origins exist in a bainite structure formed from prior austenite grains having an aspect ratio of 1.5 or less as in the present invention, the Vickers hardness of the central segregation part is suppressed. The length needs to be 300 or less. For this reason, the upper limit is set to 300. More preferably, it is 280 or less. In addition, as a measuring method of the Vickers hardness of a center segregation part, it is desirable to use the maximum value of the micro Vickers hardness measured 5 points or more with the load in which an indentation becomes smaller than a center segregation part.

Voids present in the center segregation part, MnS inclusions, inclusions made of a compound containing Nb or Ti or both, and inclusion clusters made of a compound containing Al or Ca or both have a major axis of less than 200 μm When the Vickers hardness of the central segregation part is suppressed to 300 or less, the occurrence and propagation of HIC can be suppressed if the major axis of the inclusion that becomes the starting point of HIC is less than 200 μm. Therefore, the upper limit of the major axis existing in the center segregation part is set to less than 200 μm. The HIC starting point may be voids, MnS inclusions, inclusions made of a compound containing Nb, Ti or both, and inclusion clusters made of a compound containing Al, Ca or both. For this reason, when all these exist in the center segregation part, the upper limit of a major axis shall be less than 200 micrometers.

More than 30% of the number of oxides containing Al and Ca in the steel is an oxide Al-Ca oxide having a molar ratio of Al ₂ O ₃ /CaO=0.7 to 1.3. It is agglomerated and clustered by being held in the molten steel. This cluster of Al—Ca-based oxide serves as a starting point of HIC and deteriorates the HIC resistance. In order to suppress the formation of Al—Ca-based oxide clusters, it is effective to lower the melting point of the oxide, and the compositional balance between Al ₂ O ₃ and CaO is Al ₂ O ₃ / CaO = 1 in terms of molar ratio. The clustering can be suppressed most at .0. Among oxides containing Al and Ca, when the ratio of oxides with a molar ratio of Al ₂ O ₃ /CaO=0.7 to 1.3 is less than 30% in number, Al ₂ O ₃ or CaO becomes excessive. Forms clusters in the molten steel and degrades the HIC resistance. Therefore, the 30% lower limit of the proportion of oxide containing Al and Ca in the _Al 2 _O 3 _/CaO=0.7~1.3 molar ratio. More preferably, it is 40% or more.

3. Manufacturing Conditions In the present invention, a slab obtained by continuously casting steel having the above component composition is hot forged under the following conditions. Of the steel melting methods, the Ca addition method will be described later. Moreover, all the temperature in manufacturing conditions shall be the temperature in plate | board thickness center part. The temperature at the center of the plate thickness is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the plate thickness center temperature is obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

Heating temperature Ta: 1000-1350 ° C
When forging is performed at a high temperature, it is easy to increase the rolling reduction per pass, and the diffusion of alloy elements is also promoted. On the other hand, when the heating temperature exceeds 1350 ° C., the γ grains become coarse and the toughness is deteriorated, so the upper limit is set to 1350 ° C. Further, if the temperature is lower than 1000 ° C., the reduction ratio in forging cannot be secured, and even if it is kept for a long time, the effect of diffusion of the alloy element cannot be expected.

Holding time: 300 min or more During forging heating, holding at the heating temperature Ta described above allows MnS and NbCN in the central segregation part to be dissolved, and as a result, the HIC resistance is improved. The conditions for solid solution are defined by the following formulas (3) and (4). In any of these formulas, the solid solution state of MnS or NbCN cannot be secured unless the holding time is 300 min or more. For this reason, the lower limit is set to 300 min.

Reduction ratio per pass: 5% or more It is necessary to secure a reduction ratio in hot forging for crimping an uncompressed portion called a zaku. In order to press-fit the zaku generated at the center of the slab thickness, it is necessary to reduce the center of the slab thickness, and as the rolling reduction per pass increases, the reduction applied to the center of the slab thickness increases. In order to sufficiently bond zaku or the like, the amount of reduction per pass needs to be 5% or more. For this reason, the lower limit is set to 5%.

Total reduction ratio R: 10% or more Securing of the reduction ratio in hot forging is necessary for pressure bonding of uncompressed parts called Zaku. In order to crimp the zak that occurs at the center of the slab thickness, it is necessary to reduce the center of the slab thickness.The larger the total rolling reduction, the greater the crimping effect of the zaku and so on. The total rolling reduction needs to be 10% or more. For this reason, a minimum is made into 10%.

PCCT: 1.05 or less In the present invention, the element concentrated in the center segregation part is diffused and pulverized by heating and forging pressure during hot forging to reduce the center segregation part hardness and ensure the HIC resistance. In order to suppress the desired center segregation hardness, the PCCT needs to be 1.05 or less. For this reason, PCCT shall be 1.05 or less. PCCT is expressed by the following equation (2).
PCCT = CP + MnP / 6 + 0.116Cu + 0.113Ni + 0.236Cr + 0.390Mo + 0.348V + 2PP (2)
However, CP, MnP and PP are values calculated by the following formulas (2-1) to (2-3).
CP = C + (0.77−C) erf (795 · R / 400000000 / (((0.000023exp (−17800 / (T _b +273))) 60t _b ) ^0.5 )) (2-1) )
MnP = Mn + 1.702 Mn · erf (795 / 4000,000 / (((0.00004exp (−31511 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-3)
In the above formulas (2-1) to (2-3), C, Mn, and P are set to 0 when not contained in the content (% by mass) of each element. Ta is heating temperature at the time of hot forging (℃), t _a heating holding time at the time of hot forging (min), R is the total reduction ratio during hot forging (%), T _b is the time of hot rolling the heating temperature (℃), _{t b} is the heating holding time at the time of hot rolling (min). Note that erf is an error function.

SMNS: 0 or more In order to ensure HIC resistance, it is effective to suppress the generation of elongated MnS at the center segregation part. In the present invention, HIC resistance is ensured by dissolving MnS crystallized by heating during forging. When SMNS is 0 or more, MnS in the center segregation part of the slab is dissolved, and HIC resistance can be ensured. For this reason, SMNS is 0 or more. SMNS is represented by the following formula (3).
SMNS = Ta + 273-5560 / (0.72-log [1.2Mn] [S]) (3)
However, in said formula (3), Mn and S are content (mass%) of each element, and set it as 0 when not containing. Ta is a heating temperature at the time of hot forging.

SNBCN: 0 or more In order to ensure HIC resistance, it is effective to suppress the generation of NbCN at the center segregation part. In the present invention, NbCN crystallized by heating at the time of hot forging is dissolved to ensure HIC resistance. When SNBCN is 0 or more, NbCN in the slab center segregation part can be solid-dissolved to ensure HIC resistance. For this reason, SNBCN is 0 or more. NbCN, when Ti is added to the steel, is complexed with TiN and hardly dissolves. However, by setting SNBCN to 0 or more, the cluster diameter of NbCN can be reduced, and further, excellent HIC resistance can be secured by suppressing the above-mentioned center segregation portion hardness to a certain level or less.
SNBCN is represented by the following formula (4).
SNBCN = Ta + 273-7770 / (2.26-log [5Nb] [C + 12N / 14]) (4)
However, in said Formula (4), Nb, C, and N are content (mass%) of each element, and set it as 0 when not containing. Ta is a heating temperature at the time of hot forging.

  In the present invention, the steel material hot forged under the above conditions is air-cooled, then hot-rolled under the following conditions, and further reheated and cooled.

Heating temperature _T b: _880~1300 ℃
When the heating temperature is less than 880 ° C., the structure remains in an untransformed state in the heating stage and is coarsened to lower the toughness after quenching, so the lower limit is set to 880 ° C. On the other hand, if the temperature exceeds 1300 ° C., the microstructure after quenching becomes coarse and toughness cannot be ensured, so the upper limit is set to 1300 ° C.

Holding time t _b
The longer the heating and holding time during hot rolling, the greater the effect of diffusing the alloy elements, so the longer one is preferable. On the other hand, if the holding time is shorter than 10 minutes, the slab will not be heated uniformly, resulting in large variations in strength and toughness, and the effect of improving the center segregation due to diffusion of alloy elements cannot be obtained. Therefore, the lower limit is preferably 10 min.

Reheating temperature: 880-1100 ° C
When the reheating temperature is lower than 880 ° C., the structure remains untransformed as it is rolled in the heating stage, and the toughness is lowered by coarsening, so the lower limit is set to 880 ° C. On the other hand, if the temperature exceeds 1100 ° C., the microstructure becomes coarse and toughness cannot be secured, so the upper limit is set to 1100 ° C.

Cooling start temperature: The thickness center is Ar ₃ points or more The cooling start temperature is a condition that affects the HIC resistance. Cooling is generally by water cooling. However, the present invention is not limited to this. When the cooling start temperature is lower than the Ar ₃ point, ferrite is generated, and a uniform bainite structure cannot be obtained and the HIC resistance is deteriorated. In particular, if ferrite is generated at the center of the plate thickness, the hardness of the center segregation portion increases, and the desired HIC resistance cannot be ensured. For this reason, the lower limit of the cooling start temperature is defined as Ar ₃ points.
Ar ₃ points may be actually measured, and more simply, the following formula may be used.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo
However, each element symbol is content (mass%), and is 0 when not contained.

Also, the lower the rolling temperature, the better the toughness, so the lower the rolling temperature is, the better it is within the range not lower than the Ar ₃ point. More _{preferably Ar 3 ~ (Ar 3 +50)} ℃.

Cooling rate: 4 ° C./s or more The cooling rate needs to ensure a predetermined value or more in order to obtain a uniform bainite structure. When the cooling rate is less than 4 ° C./s, ferrite is generated in the microstructure and the HIC resistance is deteriorated, so the lower limit is set to 4 ° C./s. In addition, let a cooling rate be the cooling rate in the plate | board thickness center part of a steel plate. In addition, it does not specifically limit about cooling, What is necessary is just to cool by water cooling, for example.

  In addition, in this invention, in order to obtain required intensity | strength and toughness, after air-cooling after the above-mentioned cooling, a tempering process can be performed. The reasons for the provision are described below.

Tempering temperature: 480-720 ° C
A tempering heat treatment may be performed after air cooling. Tempering is performed to improve strength and toughness, and to reduce changes in strength and toughness when performing SR (Stress Relief) heat treatment or PWHT (Post Welding Heat Treatment). At a temperature lower than 480 ° C., the tempering effect cannot be obtained, so the lower limit is set to 480 ° C. On the other hand, at temperatures exceeding 720 ° C., the strength is greatly reduced and the desired strength cannot be obtained, so the upper limit is set to 720 ° C.

  Next, the addition method of Ca is demonstrated among the melting methods of steel.

In order to control the composition ratio of Al and Ca oxides defined in the present invention within an appropriate range, it is preferable to strictly control the amount of Ca added. In the present invention, molten steel Insol. It is preferable to analyze Al and add a Ca source so that Al ₂ O ₃ / CaO in the molten steel has a molar ratio of 0.7 to 1.3 based on the analysis result. Insol. Al is insulable Al, and refers to the amount of acid-insoluble Al in the total amount of Al in steel.
In the conventional process, the Ca source is generally added in a state where the amount of Al ₂ O ₃ in the molten steel is unknown. The cause is that in order to analyze the amount of O, it is necessary to perform a combustion analysis, and the amount of O analysis was not in time until the addition of the Ca source. In the present invention, Insol. It is preferable to estimate the amount of Al ₂ O ₃ in the molten steel by analyzing the amount of Al. The aim of the Ca addition amount is that Al ₂ O ₃ : CaO is 1: 1, and in order to satisfy it, Al ₂ O ₃ / CaO in the molten steel is 0.7 to 1. It is preferable to add Ca so as to be 3.

Before adding Ca, Insol. The molten steel shown in Table 1 with a rapid analysis of Al and various changes in Al ₂ O ₃ / CaO was continuously cast at a casting speed of 0.6 mm / min (slab thickness 300 mm), and hot forging under the conditions shown in Table 2 Then, hot rolling and reheating (tempering) treatment were performed. Tempering was performed for some conditions. Ca was added using a Ca—Si wire by controlling the amount of added Ca based on the relationship between the wire weight and the Ca content.

About the obtained steel plate, the tension test and the HIC test were done.
In the tensile test, a round bar tensile test piece having a diameter of 12.7 mm was sampled from a 1/4 position in the thickness direction of the C direction, and the tensile strength was measured. The target value was 490 MPa, which is the lower limit value of ASTM-A516-65, as the lower limit value.
The HIC test was performed according to NACE TM0284-2003, and the solution A used was the same standard. In the same standard, when the plate thickness exceeds 30 mm, it is required to collect a plurality of test pieces in the plate thickness direction. Therefore, the specified number in the plate thickness direction × 3 (for example, 5 for a plate thickness of 100 mm). × 3 = 15) test was performed, and the maximum CLR (average value of three cross sections) was obtained. The target value was 15% or less in CLR. Here, CLR is a crack length ratio (CLR, total length of cracks / average value of test piece width (20 mm)).
The microstructure was obtained by etching a cross section parallel to the rolling direction of the steel sheet with a 5% nital solution and observing with an optical microscope. Since the microstructure was a structure of ferrite or bainite in all the steel plates, the fraction was measured using bainite as the part other than ferrite. The aspect ratio of the prior austenite grains was determined by observing the structure in which the prior austenite grain boundaries were exposed by picric acid corrosion with an optical microscope, and using the average aspect ratio of the 10 prior austenite grains.
The center segregation part hardness was determined by measuring the hardness at 20 points of the center segregation part with a micro Vickers with a load of 50 gf using the test piece used for the microstructure observation.
Measurements of voids in the center segregation part, inclusions made of a compound containing MnS inclusions, Nb, Ti or both, inclusions made of a compound containing Al, Ca or both are performed in the rolling direction of the steel sheet. The parallel cross section was etched with a 5% nital solution and observed with an optical microscope, and the one with the longest major diameter seen at the central segregation part was measured.

  The test results are shown in Table 3.

  In the example of the present invention, all the characteristics are satisfied, while in the comparative example, it is understood that either the tensile strength or the HIC performance is deteriorated.

Claims (4)

The component composition is C: 0.030 to 0.200%, Si: 0.50% or less, Mn: 0.60 to 1.60%, P: 0.020% or less, S: 0.005% by mass. 0015% or less, Al: 0.060% or less, Ca: 0.0010 to 0.0040%, N: 0.0050% or less, O: 0.0030% or less, and Cu: 0.50% Hereinafter, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.100% or less, Ti: 0.020% or less, B: containing 0.0030% or less of one or more, Pcm represented by the formula (1) is 0.280 or less, the balance Fe and unavoidable impurities,
From the surface layer, the microstructure of 1 / 8t (t is the plate thickness) to 7 / 8t position, bainite generated from the prior austenite grains having an aspect ratio of 1.5 or less occupies 90% or more of the total microstructure, The center segregation part has a Vickers hardness of 300 or less, and contains voids present in the center segregation part, MnS inclusions, inclusions made of a compound containing Nb or Ti, or both, Al or Ca, or both The inclusion clusters composed of the compounds all have a major axis of less than 200 μm,
30% or more of the number of oxides containing Al and Ca in steel is an oxide having a molar ratio of Al ₂ O ₃ /CaO=0.7 to 1.3. Extra heavy steel plate with excellent HIC resistance.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
However, in said formula (1), each element symbol is content (mass%), and is set to 0 when not containing. It is a manufacturing method of the extra-thick steel plate according to claim 1,
The continuous casting slab having the component composition according to claim 1 obtained by continuous casting of molten molten steel is held at a heating temperature Ta of 1000 to 1350 ° C for 300 min or more, and the rolling reduction per pass is 5% or more, total reduction ratio R is 10% or more, PCCT represented by formula (2) is 1.05 or less, SMNS represented by formula (3) is 0 or more, and SNBCN represented by formula (4) is 0 or more. Air forging after hot forging under the conditions
Then, after reheating at a heating temperature T _{b of} 880 to 1300 ° C. and a holding time t _b and performing hot rolling, air cooling and further reheating to a temperature of 880 ° C. to 1100 ° C. Is cooled at a cooling rate of 4 ° C./s or more from a temperature range of ₃ or more points of Ar, and a method for producing a very thick steel plate having a plate thickness of 50 mm or more and excellent HIC resistance.
PCCT = CP + MnP / 6 + 0.116Cu + 0.113Ni + 0.236Cr + 0.390Mo + 0.348V + 2PP (2)
SMNS = T _a + 273-5560 / (0.72-log [1.2Mn] [S]) (3)
SNBCN = T _a + 273-6770 / (2.26-log [5Nb] [C + 12N / 14]) (4)
However, CP, MnP and PP are values calculated by the following formulas (2-1) to (2-3). In the above formulas (2) to (4), Cu, Ni, Cr, Mo, V, Mn, Nb, C, and N are the contents (mass%) of each element, and 0 when not contained. To do. Ta is the heating temperature during hot forging.
CP = C + (0.77−C) erf (795 · R / 400000000 / (((0.000023exp (−17800 / (T _b +273))) 60t _b ) ^0.5 )) (2-1) )
MnP = Mn + 1.702 Mn · erf (795 / 4000,000 / (((0.00004exp (−31511 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (Ta + 273))) 60 t _a ) ^0.5 )) (2-3)
However, in the above formulas (2-1) to (2-3), C, Mn, and P are set to 0 when not contained in the content (mass%) of each element. Ta is heating temperature at the time of hot forging (℃), t _a heating holding time at the time of hot forging (min), R is the total reduction ratio during hot forging (%), T _b is the time of hot rolling the heating temperature (℃), _{t b} is the heating holding time at the time of hot rolling (min). Note that erf is an error function.   The method for producing an extra-thick steel plate with excellent HIC resistance having a thickness of 50 mm or more according to claim 2, wherein air cooling is performed after the cooling and tempering heat treatment is performed in a temperature range of 480 to 720 ° C. Insol. The aluminum is analyzed, and Ca is added so that Al ₂ O ₃ / CaO in the molten steel is 0.7 to 1.3 in a molar ratio based on the analysis result. 3. A method for producing an ultra-thick steel plate having excellent HIC resistance and having a thickness of 50 mm or more according to 3.