JP2017048443A - Extra thick steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an extra thick steel sheet having excellent internal quality and HIC resistance performance even with sheet thickness of 50 mm or more, and a manufacturing method capable of manufacturing the extra thick steel sheet at high productivity.SOLUTION: An extra thick steel sheet excellent in HIC resistance performance has a specific component composition. The extra thick steel sheet has a micro structure having area percentage of ferrite, pearlite and bainite of 95% or more in total, hardness of center segregation part of 300 or less, major axis of gap existing in the center segregation part, a MnS-based inclusion, an inclusion consisting of Nb and/or Ti and an inclusion cluster consisting of Al and/or Ca of less than 200 μm, number percentage of oxide with AlO/CaO in oxide containing Al and Ca of 0.7 to 1.3 by molar ratio of 30% or more, and sheet thickness of 50 mm or more.SELECTED DRAWING: None

Description

  The present invention relates to a very thick steel plate used in a wet hydrogen sulfide environment typified by pressure vessels such as petroleum refining planners and process pipes, and a method for producing the same, and particularly, excellent HIC resistance (Hydrogen Induced Cracking). The present invention relates to an extra-thick steel plate used for a member that requires performance and a method for manufacturing the same.

  Crude oil extraction 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 plate having a plate thickness of 50 mm or more, the target performance cannot always be 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 plate that achieves both excellent HIC resistance and internal quality is disclosed by performing diffusion, reheating, and applying optimal TMCP (Thermo Mechanical Control Process) conditions. 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 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, the methods disclosed in Patent Documents 1 and 2 both 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, in the method disclosed in Patent Document 3, 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. In this specification, ensuring sufficient internal quality means that not only the vicinity of the surface of the steel material but also the vicinity of the center part in the thickness direction of the steel material substantially eliminates cavities and defects such as zaku and porosity. It means reducing to a harmless level.

  As described above, with the conventional technology, it is difficult to produce a very thick steel plate with excellent HIC resistance that does not cause a decrease in productivity, deterioration in internal quality, and deterioration in HIC resistance caused thereby. Therefore, an object of the present invention is to provide a very thick steel plate having excellent internal quality and HIC resistance even if the plate thickness is 50 mm or more, and a manufacturing method capable of manufacturing the very thick steel plate with high productivity. To do.

  In order to solve the above-mentioned problems, the present inventors have developed forging conditions that affect the internal quality of the slab, chemical components (component composition) that affect the center segregation hardness and inclusions, forging conditions, and rolling. As a result of intensive studies, the following findings were obtained.

  First, as a result of examining forging conditions that affect the internal quality of continuous cast slabs, zaku and porosity were pressure-bonded by securing a reduction rate per pass of 5% or more and a total reduction rate R of 10% or more. It was found that a rolled material excellent in internal quality can be obtained, and as a result, excellent HIC resistance can be obtained after hot rolling.

  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 idea, SMNS (formula (3) described later) and SNBCN (formula (4) described later) were devised. When both of these become 0 or more, most of MnS and NbCN in the central segregation part can be dissolved, and these are prevented from becoming the starting point of 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, PCCT was proposed as an index of central segregation hardness. PCCT 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). When manufactured by normalization, by setting this value to 0.95 or less, the hardness of the central segregation part can be lowered, and there are fine inclusions having a size of less than 200 μm and their accumulation bands. It was also found that excellent performance can be obtained in the HIC test of the NACE TM0284-A solution. By using PCCT, 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 completed based on the above findings, and is specifically as follows.

[1] By mass%, 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 %: 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 type or two or more types, Pcm represented by the formula (1) satisfies 0.280 or less, the balance is composed of Fe and unavoidable impurities, ferrite, pearlite and The area ratio of bainite is 95% or more in total, and the bit at the center segregation part is The hardness of the source is 300 or less, and the major axis of the voids present in the central segregation part, MnS inclusions, inclusions consisting of Nb and / or Ti, inclusion clusters consisting of Al and / or Ca is less than 200 μm And a microstructure in which the number ratio of the oxide having an Al ₂ O ₃ / CaO molar ratio of 0.7 to 1.3 in the oxide containing Al and Ca is 30% or more, and the plate thickness is 50 mm Extra heavy steel plate with excellent HIC resistance characterized by the above.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
The element symbol in the formula (1) means the content (% by mass) of each element, and 0 is not included.

[2] A continuous casting slab composed of the component composition according to [1] obtained by continuously casting molten steel is heated at 1000 to 1350 ° C. at a heating temperature T _a (° C.) and at least 300 minutes. Reheating at t _a (min), the rolling reduction per pass is 5% or more, the total rolling reduction R is 10% or more, the PCCT represented by the formula (2) is 0.95 or less, and the formula (3) After performing hot forging under the condition that the SMNS is 0 or more and the SNBCN represented by the formula (4) is 0 or more, air cooling is performed, and then a heating temperature T _{b of} 880 to 1300 ° C. and a heating holding time t of 10 min or more. _{b. Reheating} at _b , hot rolling, air cooling, and further reheating to a temperature of 880 ° C. or higher and 1100 ° C. or lower, followed by air cooling. An excellent method for manufacturing extremely thick steel plates.
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). Moreover, in said formula (2)-(4), Cu, Ni, Cr, Mo, V, Mn, S, Nb, C, and N are content (mass%) of each element, and when not containing 0. 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 / (T _a +273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (T _a +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 according to [2], wherein a tempering heat treatment is performed in a temperature range of 480 to 720 ° C. after air cooling after reheating to a temperature of 880 ° C. or more and 1100 ° C. or less.

[4] Insol. Analyzing Al, and adding Ca 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 [2] Or the manufacturing method of the extra-thick steel plate as described in [3].

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

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

  The extra-thick steel plate of the present invention is, in mass%, 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: 0.00. A component composition containing 020% or less, B: 0.0030% or less, Pcm represented by the formula (1) satisfying 0.280 or less, and the balance being Fe and inevitable impurities Have. In the following description of the component composition, “%” means “% by mass”.

C: 0.030 to 0.200%
C is inexpensive and contributes to high strength. On the other hand, C is an element that deteriorates the degree of central segregation. From the viewpoint of securing the HIC resistance, it is better to reduce the content thereof. However, if the C content is lower than 0.030%, the hardenability becomes too low to obtain the desired strength, so the lower limit is made 0.030%. On the other hand, when the C content exceeds 0.200%, the degree of segregation deteriorates and the HIC resistance cannot be ensured, 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 inevitably contained in the slab. Further, Si is an element that can be increased in strength without taking into consideration that the hardenability of the center segregation portion is increased. If the Si content 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, Mn tends to concentrate in the center segregation part, and deteriorates the HIC resistance. If the Mn content exceeds 1.60%, the occurrence of HIC cannot be sufficiently 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 content 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 enhances hardenability. However, P 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 its content as much as possible, reducing the P content requires a great deal of cost. Moreover, in this invention, the concentration of P in the center segregation part at the time of forging heating can be relieved, and if the P content is 0.020% or less, the HIC resistance performance can be secured by the effect. Therefore, the upper limit of the P content is 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. Further, since S forms MnS and becomes the starting point of HIC, it adversely affects the HIC resistance performance. Therefore, it is better to reduce the S content as much as possible. However, strengthening desulfurization leads to an increase in cost. For this reason, the content of S is allowed up to 0.0015%, which is the upper limit at which the effect of suppressing the formation of MnS by adding Ca can be expected. 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. When the Al content 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. Since the effect does not appear unless the Ca content is 0.0010% or more, the lower limit of the Ca content is set to 0.0010%. On the other hand, when the Ca content exceeds 0.0040%, CaO and CaOS are excessively generated, and a cluster is formed to serve as an HIC starting point, resulting in a deterioration in anti-HIC performance. Therefore, the upper limit of the Ca content is set to 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 the N content exceeds 0.0050%, the crystallized TiN 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 of the N content is set to 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 O content 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 and the HIC resistance cannot be secured, so the upper limit is made 0.280%. More preferably, Pcm 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 Cu, the upper limit of content shall be 0.50%. In order to increase the strength, the Cu content is preferably 0.10% or more.

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 Ni, the upper limit of content shall be 1.00%. In order to increase the strength and the like, the Ni content is preferably 0.10% or more.

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 Cr, the upper limit of content is made into 0.50%. In order to increase the strength, the Cr content is preferably 0.10% or more.

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 Mo, the upper limit of content shall be 0.50%. In order to increase the strength, the Mo content is preferably 0.05% or more.

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 the Nb content exceeds 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 Nb, the upper limit of the content is made 0.050%. More preferably, it is 0.040% or less. In order to refine the structure, the Nb content is preferably 0.005% or more.

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 V, the upper limit of the content is made 0.100%. In order to increase the strength, the V content is preferably 0.010% or more.

Ti: 0.020% or less Ti refines the structure by forming TiN. In particular, Ti 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 Ti, the upper limit of the content shall be 0.020%. Note that the Ti content is preferably 0.005% or more in order to refine the structure.

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 the B content exceeds 0.0030%, the hardenability becomes too high, the hardness of the steel sheet surface layer and the weld heat affected zone increases, and the SSC resistance is deteriorated. For this reason, when it contains B, the upper limit of content is made into 0.0030%. In order to increase the strength, the B content is preferably 0.0005% or more.

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

Subsequently, the microstructure of the extra-thick steel plate of the present invention will be described. In the microstructure of the present invention, the total area ratio of ferrite, pearlite, and bainite is 95% or more, the Vickers hardness of the center segregation part is 300 or less, voids present in the center segregation part, MnS inclusions, The major axis of the inclusion cluster composed of Nb and / or Ti, the inclusion cluster composed of Al and / or Ca is less than 200 μm, and Al ₂ O ₃ / CaO in the oxide containing Al and Ca has a molar ratio of 0.7 to The number ratio of the 1.3 oxide is 30% or more. Hereinafter, the microstructure will be specifically described.

Total of ferrite, pearlite and bainite: 95% or more Since the extra-thick steel sheet of the present invention is manufactured by normalization, the microstructure inevitably contains ferrite. Ferrite is the first phase in the microstructure, and its content is preferably 60 to 95% in terms of area ratio. From the viewpoint of securing the HIC resistance, it is not desirable to have a hard microstructure. In the present invention, as a phase other than ferrite, it is necessary to include a soft phase such as pearlite and a relatively hard but tough phase such as bainite. The pearlite content is preferably 5 to 40% by area ratio, and the bainite content is preferably 40% or less by area ratio. Therefore, in the present invention, the total area ratio of ferrite, pearlite, and bainite is 95% or more. In the present invention, at least one of pearlite and bainite may be included. When other than these are included in an area ratio exceeding 5%, HIC is generated and propagated and the HIC resistance is deteriorated. As phases other than ferrite, pearlite and bainite, it means martensite and coarse cementite, 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 that are the origin of HIC such as MnS, NbCN, TiN, Al ₂ O ₃ , and CaOS are generated. In the present invention, MnS and NbCN can be dissolved by heating during forging. However, since TiN, Al ₂ O ₃ , and CaOS do not form a solid solution, these serve as starting points for generating HIC. Even when these starting points exist, in order to suppress the generation and propagation of HIC, it is necessary to make the Vickers hardness of the central segregation part 300 or less. Therefore, the upper limit is set to 300. The Vickers hardness of the center segregation part is more preferably 280 or less. As a method for measuring the segregation hardness, it is desirable to use the maximum value of micro Vickers hardness measured at 5 or more points with a load that makes the indentation smaller than the segregation part.

Long diameter of voids and the like existing in the center segregation part: less than 200 μm When the Vickers hardness of the center segregation part is suppressed to 300 or less, generation of HIC occurs if the long diameter of voids and inclusions that are the starting point of HIC is less than 200 μm , Can suppress the propagation. Therefore, the upper limit of the major axis is set to 200 μm. Here, the voids and the like serving as HIC starting points are voids, MnS inclusions, inclusions composed of Nb, Ti or both, and inclusion clusters composed of Al, Ca, or both. When these are present in the center segregation part, all the major diameters need to be less than 200 μm when confirmed by the method described in the examples.

Al ₂ O ₃ / CaO is a ratio of the number of oxides having a molar ratio of 0.7 to 1.3: 30% or more Al—Ca-based oxides are formed in molten steel and aggregated by being retained in the molten steel. To cluster. 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 Al ₂ O ₃ and CaO have a molar ratio (Al ₂ O ₃ / CaO) of 1.0. Sometimes clustering can be most suppressed. Among the oxides containing Al and Ca, when the number ratio of the oxide having an Al ₂ O ₃ / CaO molar ratio of 0.7 to 1.3 is less than 30%, Al ₂ O ₃ or CaO becomes excessive and the molten steel A cluster is formed therein, and the HIC resistance is deteriorated. Therefore, _Al 2 _O 3 / CaO in the oxide containing Al and Ca is the number of oxides of 0.7 to 1.3 with 30% or more by molar ratio. More preferably, it is 40% or more.

Then, the manufacturing method of the extra-thick steel plate of this invention is demonstrated. The extra-thick steel plate of the present invention reheats a continuously cast slab having the above composition at a heating temperature T _a (° C.) of 1000 to 1350 ° C. and a heating holding time t _a (min) of 300 min or more per pass. After performing hot forging under conditions where the rolling reduction ratio is 5% or more, the total rolling reduction R is 10% or more, PCCT is 0.95 or less, SMNS is 0 or more, and SNBCN is 0 or more, After reheating at a heating temperature T _{b of} 880 to 1300 ° C. and a heating holding time t _b of 10 min or more, hot rolling, air cooling, and further reheating to a temperature of 880 ° C. or more and 1100 ° C. or less It can be manufactured by air cooling. Hereinafter, manufacturing conditions will be described.

Heating temperature T _a during hot forging: 1000 to 1350 ° C.
If forging is performed at a high temperature, the rolling reduction per pass can be easily increased, and the diffusion of alloy elements can be promoted. On the other hand, the heating temperature T _a is lead by weight, the γ grains are deteriorated in toughness coarsened the 1350 ° C.. Therefore, the 1350 ° C. The upper limit of the heating temperature _{T a.} Furthermore, can not be secured rolling reduction in forging and a heating temperature T _a is below 1000 ° C., it can not be expected the effect of diffusion of the alloying elements be held for a long time. Therefore, the lower limit of the heating temperature _{T a} and 1000 ° C..

Heating retention time _t a during the hot forging: the high temperature holding of 300min or more forging heating, HIC resistance is improved by making a solid solution of MnS and NbCN of center segregation. The conditions for solid solution are defined by equation (3) described later and equation (4) described later. Since these equations also heated holding time eventually _{t a} is unable to ensure the solid solution state of the MnS or NbCN not more than 300 min, the lower limit and 300 min. It is preferable heating holding time _{t a} is less 1440min from the viewpoint of productivity.

Rolling rate per pass during hot forging: 5% or more Securing the rolling rate in forging is necessary for crimping such as 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 crimp zaku or the like, the rolling reduction per pass needs to be 5% or more, so the lower limit is set to 5%.

Total reduction ratio R in hot forging: 10% or more Securement of the reduction ratio in hot forging is necessary for pressure bonding of uncompressed parts called 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. The larger the total rolling reduction R is, the greater the effect of crimping zaku etc. is. In order to sufficiently crimp zaku etc., the total rolling reduction R needs to be 10% or more. For this reason, the lower limit of the total rolling reduction R is set to 10%. The upper limit of the total rolling reduction R is not particularly limited. However, if the total rolling reduction R is too large, shape control becomes difficult at the subsequent hot rolling stage, resulting in a decrease in yield.

PCCT: 0.95 or less In the present invention, the element concentrated in the center segregation is diffused and pulverized by heating and forging pressure during forging to reduce the center segregation hardness and ensure the HIC resistance. In order to suppress the center segregation hardness to a desired value, the PCCT represented by the formula (2) needs to be 0.95 or less.
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). Moreover, in said formula (2), Cu, Ni, Cr, Mo, and V are content (mass%) of each element, and set it as 0 when not containing.
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 / (T _a +273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (T _a +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. T _a is the heating temperature (° C.) during hot forging, t _a is the heating holding time (min) during hot forging, R is the total reduction ratio (%) during hot forging, and T _b is during hot rolling. The heating temperature (° C.) and t _b are the heating holding time (min) during hot rolling. Note that erf is an error function.

SMNS: 0 or more In order to ensure anti-HIC performance, it is effective to suppress the generation of elongated MnS due to center segregation. In the present invention, HIC resistance is ensured by dissolving MnS crystallized by heating during forging. When the SMNS represented by the formula (3) is 0 or more, the slab center segregated MnS is dissolved and does not deteriorate the HIC resistance.
SMNS = T _a + 273-5560 / (0.72-log [1.2Mn] [S]) (3)
In said Formula (3), Mn and S are content (mass%) of each element, and set it as 0 when not containing. Further, T _a is the heating temperature at the time of hot forging. “Log [1.2Mn] [S]” means log ([1.2Mn] × [S]).

SNBCN: 0 or more In order to ensure anti-HIC performance, 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 = T _a + 273-6770 / (2.26-log [5Nb] [C + 12N / 14]) (4)
In said formula (4), Nb, C, and N are content (mass%) of each element, and set it as 0 when not containing. Further, T _a is the heating temperature at the time of hot forging. Note that “log [5Nb] [C + 12N / 14]” means log ([5Nb] × [C + 12N / 14]).

  After performing the above hot forging, air cooling is performed. Air cooling means cooling to a temperature range of 200 ° C. or less at a cooling rate of 1 ° C./s or less.

Heating temperature T _{b at} the time of hot rolling: 880 to 1300 ° C.
Heating temperature T _b at the time of hot rolling, a condition for controlling the normalizing previous tissue. When the heating temperature _Tb is less than 880 ° C., the toughness after normalization is lowered because the structure remains untransformed in the heating stage and remains coarse. Therefore, the lower limit of the heating temperature _{T b} and 880 ° C.. On the other hand, the heating temperature T _b is the microstructure after normalizing baked exceeds 1300 ° C. can not secure the toughness becomes coarse. Therefore, the heating temperature _Tb is set to 1300 ° C. or lower.

Heat holding time t _b at the time of hot rolling: 10 min or longer The heat holding time t _b at the time of hot rolling is preferably longer because the diffusion effect of the alloy element becomes larger as it is longer. On the other hand, the heating retention time t _b is not heated slab uniformly below the 10min intensity, variation in toughness increases, and not be obtained the effect of improving the center segregation caused by the diffusion of alloying elements. Therefore, the lower limit is 10 min. The heating retention time _{t b} from the viewpoint of productivity, less preferred 720Min.

  After the hot rolling, air cooling is performed. Since the meaning of “air cooling” is the same as “air cooling” after the hot forging, the description thereof is omitted.

Normalizing heating temperature: 880 ° C. or higher and 1100 ° C. or lower Reheating to 880 ° C. or higher and 1100 ° C. or lower after the hot rolling corresponds to heating for normalizing. Normalizing heating temperature is a condition for controlling the structure. When the normalized heating temperature is less than 880 ° C., the structure remains untransformed as it is rolled in the heating stage, and the toughness decreases due to coarsening. For this reason, the lower limit of the normalizing heating temperature is set to 880 ° C. On the other hand, if the normalizing heating temperature exceeds 1100 ° C., the microstructure becomes coarse and toughness cannot be ensured. For this reason, the upper limit of the normalizing heating temperature is set to 1100 ° C.

  After reheating for normalization, air-cool. Since the meaning of “air cooling” is the same as “air cooling” after the hot forging, the description thereof is omitted.

  In the present invention, tempering can be performed after normalization in order to obtain the required strength and toughness. 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 to bring Al ₂ O ₃ : CaO close to 1: 1, and in order to satisfy it, Al ₂ O ₃ / CaO in the molten steel has a molar ratio of 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, air-cooling, air-cooling, heating and hot rolling, air-cooling, normalizing, air-cooling, and subsequent tempering as necessary. 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. Ca yield was added as 0.3% based on past data.

  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 450 MPa, which is the lower limit value of ASTM-A516-65.

  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 × 3 at the plate thickness of 100 mm). = 15) The test was carried out, and the maximum CLR (average value of three cross sections) was obtained. The target value was 15% or less in CLR.

  The microstructure was obtained by etching the steel plate L direction (rolling direction) cross section with 5% nital and then obtaining the ferrite fraction with an optical microscope. Moreover, the part other than a ferrite phase was observed with the electron microscope, and it classified into three types, pearlite, bainite, and other structures (cementite, MA, etc.), and calculated | required the fraction of pearlite and bainite. The observation position was performed at the center segregation part in consideration of the correlation with HIC.

  The center segregation hardness was determined by measuring the 20-point Vickers hardness of the center segregation part in a micro Vickers hardness test with a load of 50 g using the specimen used for the microstructure observation, and using the highest value.

  To measure the major axis of voids at the center segregation part, inclusions made of MnS inclusions, inclusions made of Nb, Ti or both, inclusion clusters made of Al, Ca or both, the steel sheet L direction section was etched with 5% nital. And by observing with an optical microscope. The one with the longest major axis seen at the center segregation part was measured.

The Al ₂ O ₃ / CaO ratio in the oxide is obtained by observing a mirror-polished sample with an electron microscope and performing inclusion composition analysis on the spherical inclusion by EDX (energy dispersive X-ray analysis). It was. The composition ratio of inclusions was measured for 100 or more per steel plate in consideration of variation, and the average value was obtained.

  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)

In mass%, 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, 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: 0.00. A component composition containing one or more of 0030% or less, Pcm represented by the formula (1) satisfying 0.280 or less, and the balance comprising Fe and inevitable impurities;
The area ratio of ferrite, pearlite and bainite is 95% or more in total, the Vickers hardness of the center segregation part is 300 or less, and consists of voids, MnS inclusions, Nb and / or Ti present in the center segregation part. Inclusion, the major axis of the inclusion cluster composed of Al and / or Ca is less than 200 μm, and the oxide containing Al and Ca contains Al ₂ O ₃ / CaO in a molar ratio of 0.7 to 1.3. Having a microstructure with a number ratio of 30% or more,
An ultra-thick steel plate with excellent HIC resistance, characterized in that the plate thickness is 50 mm or more.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
The element symbol in the formula (1) means the content (% by mass) of each element, and 0 is not included. A continuous casting slab comprising the component composition according to claim 1 obtained by continuously casting molten steel, a heating temperature T _a (° C.) of 1000 to 1350 ° C., a heating holding time t _a (300 min or more). min)), the reduction rate per pass is 5% or more, the total reduction rate R is 10% or more, the PCCT represented by the equation (2) is 0.95 or less, and the SMNS represented by the equation (3) is After performing hot forging under the condition that 0 or more and SNBCN represented by formula (4) is 0 or more, air cooling,
Then, after reheating at a heating temperature T _{b of} 880 to 1300 ° C. and a heating holding time t _b of 10 min or longer, hot rolling is performed, and then air-cooled, and further reheated to a temperature of 880 ° C. or higher and 1100 ° C. or lower. After that, a method for producing an extra-thick steel plate having excellent HIC resistance with a thickness of 50 mm or more, which is air-cooled.
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). Moreover, in said formula (2)-(4), Cu, Ni, Cr, Mo, V, Mn, S, Nb, C, and N are content (mass%) of each element, and when not containing 0. 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 / (T _a +273))) 60 t _a ) ^0.5 )) (2-2)
PP = P / 10.18P · erf (795 / 4000,000 / (((0.00087exp (−0.4406 / (T _a +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 according to claim 2, wherein tempering heat treatment is performed in a temperature range of 480 to 720 ° C after air cooling after reheating to a temperature of 880 ° C to 1100 ° C. Insol. The aluminum is analyzed, and Ca is added 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. Or the manufacturing method of the extra-thick steel plate of 3.