JP5687946B2 - High strength thick steel plate with excellent toughness - Google Patents

Description

  The present invention relates to a high-strength thick steel plate (hereinafter sometimes referred to as “570 MPa class thick steel plate”) having a tensile strength of 570 MPa or more, which is suitably used for large structures such as building structures and bridges. In particular, the present invention relates to a high-strength thick steel plate with reduced variation in toughness.

  As for thick steel plates used in the fields of construction, shipbuilding, bridges, etc., application of high strength thick steel plates with a tensile strength of 570 MPa class is spreading due to the increase in size of structures. On the other hand, in addition of alloy elements for the purpose of ensuring strength, there are component restrictions such as carbon equivalent Ceq, and it is necessary to make the steel sheet strength high under such restrictions. For these reasons, so-called microalloy elements such as Nb and V, which can greatly improve the strength with a small amount of addition, are excellent in cost-effectiveness, and there is a demand for measures to make maximum use of these microalloy elements. .

  However, although the addition of the above microalloy element is effective in improving the strength of the steel sheet, it tends to increase the austenite recrystallization temperature, resulting in coarsening of crystal grains and reducing the toughness of the steel sheet. There are drawbacks. For these reasons, it is desired to establish a technique capable of improving both strength and toughness while effectively using a microalloy element. The “strength” here includes not only the “tensile strength” but also the “yield point”.

  Various techniques for ensuring good toughness as well as high strength in a 570 MPa class thick steel sheet have been proposed so far. For example, Patent Document 1 discloses a technique that appropriately defines the chemical component composition and appropriately controls the aspect ratio of the prior austenite crystal grain size to reduce acoustic anisotropy and improve strength and toughness. Has been proposed. Patent Document 2 proposes a technique for improving the properties of steel materials such as strength and toughness by appropriately defining the chemical composition and controlling the production conditions to refine the structure. Has been.

  On the other hand, in Patent Documents 3 and 4, etc., in the component system in which the C content is reduced to about 0.06% or less, by adding carbide-forming elements such as Mo, Nb, and V, the weldability of the steel sheet (HAZ toughness). In addition, a technique has been proposed that can ensure the weld crack resistance) and exhibit the characteristics (strength and toughness) of an excellent weld base material (steel plate).

  In these techniques, the strength of the steel sheet is basically excellent and the average value of toughness is excellent, but there may be variations in this characteristic. In particular, in terms of toughness, the average value satisfies the specification as well as the average value, but in some cases, the minimum value of the toughness variation may not have a margin for the specification. It is necessary to be able to reduce variations in characteristics while ensuring such a toughness value.

  On the other hand, as a technique for improving the characteristics of a steel sheet, Patent Document 5 discloses that a steel material having a predetermined chemical composition is reheated to 1100 to 1350 ° C., and the strain rate at 1000 ° C. or higher is 0.05 to 3 / A technique has also been proposed in which casting defects such as center porosity are reduced and the ductility in the sheet thickness direction is improved by performing hot working with a cumulative reduction amount of 15% or more per second. This technique is not made from the viewpoint of improving the structure of the steel sheet, but is a useful technique from the viewpoint of improving the reliability of the steel sheet.

JP 2006-283126 A JP 2007-32230 A Japanese Patent No. 3863413 Japanese Patent No. 4220871 JP 2010-106298 A

  The present invention has been made under such circumstances, and an object of the present invention is to provide a high-strength thick steel plate having excellent toughness as well as an average value as well as a minimum value.

  The high-strength thick steel plate of the present invention that has solved the above problems is C: 0.01 to 0.07% (meaning mass%, hereinafter the same for chemical composition), Si: 0.5% or less (0% Mn: 1 to 1.7%, P: 0.015% or less (not including 0%), S: 0.006% or less (not including 0%), Cr: 0.8 to 2%, V: 0.1% or less (excluding 0%), Nb: 0.005 to 0.05%, B: 0.005% or less (not including 0%), Ti: 0.005 0.02%, Al: 0.2% or less (not including 0%), Ca: 0.0035% or less (not including 0%), and N: 0.003 to 0.006%, respectively The balance is iron and inevitable impurities, 90% by area or more of the steel structure is bainite, and the average equivalent circle diameter of the prior austenite grains is 5 μm. Above, with at most 95 .mu.m, the maximum equivalent circle diameter of prior austenite grains is equal to or is 150μm or less.

  The high-strength thick steel plate of the present invention may further include (a) Mo: 0.5% or less (not including 0%), (b) Ni: 0.5% or less (not including 0%) as necessary. ) And / or Cu: 0.5% or less (not including 0%) is also useful, and the properties of the thick steel plate are further improved depending on the type of element to be contained.

  According to the present invention, by appropriately controlling the production conditions together with the chemical component composition and appropriately controlling the structure of the steel material and the form of the prior austenite grains, it is possible to reduce the variation in toughness while ensuring high strength. High-strength steel plates that can be used are realized, and such steel plates are extremely useful as materials for large structures such as building structures and bridges.

  The present inventors have studied from the viewpoint of positively adding elements such as Cr, V, Nb, and B to improve the strength of the steel sheet and improving the toughness reduction due to the addition of these elements. As a result, after heating at a predetermined temperature prior to hot rolling, if processing such as reduction at a reduction rate of 10% or more (reduction treatment) is performed, the austenite crystal grains after hot rolling are finely uniform. As a result, the inventors have found that a high-strength thick steel sheet having excellent toughness not only in the average value but also in the minimum value can be realized, and the present invention has been completed.

  It is known that the above-described reduction process (this process may be referred to as “BD process” below) is performed for other purposes (Patent Document 5). In the present invention, by applying such BD treatment as a means for improving toughness, a part of austenite crystal grains is prevented from becoming coarse during subsequent hot rolling (the fineness of austenite crystal grains after hot rolling is reduced). In the end, a high-strength thick steel sheet having excellent toughness not only in the average value but also in the minimum value was obtained. In particular, in the case of a thick steel plate containing an element that has a large effect of increasing the recrystallization temperature such as Nb, the minimum value of variation in toughness while ensuring strength can be achieved only by controlling the chemical composition and hot rolling conditions. The effect of raising the bottom is difficult, and the BD processing as described above is an essential requirement.

  The steel plate according to the present invention finally has a structure mainly composed of bainite. As a result of the fine homogenization of the austenite crystal grains after hot rolling, the average equivalent circle diameter of the prior austenite grains is 5 μm. As described above, the requirement that the maximum equivalent circle diameter of the prior austenite grains is 150 μm or less is satisfied while being 95 μm or less.

  In addition, prior austenite grains (sometimes referred to as “old γ grains”) generally means that when the structure is cooled from the austenite state, phase transformation occurs and becomes another structure such as ferrite or martensite. However, the term indicating the austenite grains before transformation from the viewpoint of the steel sheet after transformation is “old austenite grains”. The “average equivalent circle diameter” means the average value of the diameter (equivalent circle diameter) when the old γ grains are converted into a circle of the same area, and the “maximum equivalent circle diameter” is as described above. This is the maximum diameter (equivalent circle diameter) when converted.

[The average equivalent circle diameter of the old γ grains is 5 μm or more and 95 μm or less]
The thick steel plate of the present invention satisfies the requirement that the average equivalent circle diameter of the old γ grains is 5 μm or more and 95 μm or less. When the average equivalent circle diameter is less than 5 μm, ferrite transformation is promoted during the formation of the structure, the ferrite fraction increases, and a structure mainly composed of bainite (90 area% or more) cannot be obtained. That is, in a steel sheet in which the average equivalent circle diameter of the old γ grains is less than 5 μm, a structure mainly composed of bainite cannot be obtained. Further, when the average equivalent circle diameter of the old γ grains is larger than 95 μm, the final structure size becomes coarse and the toughness decreases. The preferable lower limit of the average equivalent circle diameter is 10 μm or more (more preferably 20 μm or more, more preferably 30 μm or more, further preferably 40 μm or more, particularly preferably 50 μm or more), and the preferable upper limit is 80 μm or less (more preferably). 70 μm or less).

[Maximum equivalent circle diameter of old γ grains is 150 μm or less]
The thick steel plate of the present invention satisfies the requirement that the maximum equivalent circle diameter of the old γ grain is 150 μm or less. This requirement is a requirement necessary for ensuring the minimum required toughness value. If the maximum equivalent circle diameter exceeds 150 μm, the minimum required toughness value cannot be ensured. The preferable upper limit of the maximum equivalent circle diameter is 130 μm or less (more preferably 100 μm or less).

  90% by area or more of the structure of the high-strength thick steel plate of the present invention is bainite. By setting the bainite fraction to 90% by area or more, it becomes possible to ensure the strength (particularly the yield point) of the steel sheet. The bainite fraction is preferably 95 area% or more, more preferably 97 area% or more, and particularly preferably 100 area%. As a structure other than the bainite structure, a mixed structure (MA structure) composed of martensite and austenite, ferrite, pearlite, or the like may be included in part.

  Next, the chemical component composition of the high-strength thick steel plate of the present invention will be described. In the present invention, it is also an important requirement to appropriately adjust the chemical component composition (C, Si, Mn, P, S, Cr, V, Nb, B, Ti, Al, Ca and N). The effects of these components and the reasons for setting the range are as follows.

[C: 0.01 to 0.07%]
C is an important element in securing the strength of the steel sheet. In order to exert such effects, the C content needs to be 0.01% or more. However, if the C content becomes excessive and exceeds 0.07%, the strength increases and martensite is easily generated, so that the toughness is lowered. The preferable lower limit of the C content is 0.02% or more (more preferably 0.03% or more), and the preferable upper limit is 0.06% or less (more preferably 0.05% or less).

[Si: 0.5% or less (excluding 0%)]
Si is an important element in securing the strength of the steel sheet. However, when the Si content is excessive and exceeds 0.5%, the formation of a hard MA structure (mixed structure composed of martensite and austenite) is promoted, which adversely affects toughness. The upper limit with preferable Si content is 0.45% or less (more preferably 0.4% or less). In addition, the minimum with preferable Si content for exhibiting the said effect is 0.05% or more (more preferably 0.1% or more).

[Mn: 1 to 1.7%]
Mn is an element necessary for improving hardenability, suppressing ferrite formation, and ensuring strength. If the Mn content is less than 1%, the strength is insufficient. On the other hand, when the Mn content exceeds 1.7% and becomes excessive, the toughness is reduced due to the increase in strength. A preferable lower limit of the Mn content is 1.1% or more (more preferably 1.2% or more), and a preferable upper limit is 1.6% or less (more preferably 1.5% or less).

[P: 0.015% or less (excluding 0%)]
P is an impurity element that causes grain boundary fracture. When the amount of P is excessive, the toughness deteriorates, so the P content needs to be suppressed to 0.015% or less. The P content is preferably 0.01% or less (more preferably 0.008% or less). In addition, P is an impurity inevitably contained in the steel material, and it is difficult to make the amount 0% in industrial production.

[S: 0.006% or less (excluding 0%)]
S is an impurity element that causes grain boundary fracture as in the case of P. If the amount of S is excessive, the toughness deteriorates, so the S content must be suppressed to 0.006% or less. The S content is preferably 0.005% or less (more preferably 0.003% or less). In addition, S is an impurity inevitably contained in the steel material, and it is difficult to make the amount 0% in industrial production.

[Cr: 0.8-2%]
Cr is an element necessary for suppressing the formation of ferrite and ensuring a balance between strength and toughness. If the Cr content is less than 0.8%, the strength will be insufficient. However, when the Cr content exceeds 2% and becomes excessive, the toughness is reduced due to the increase in strength. The preferable lower limit of the Cr content is 1% or more (more preferably 1.2% or more), and the preferable upper limit is 1.8% or less (more preferably 1.6% or less).

[V: 0.1% or less (excluding 0%)]
V, like Cr, is an element necessary for suppressing the formation of ferrite and ensuring a balance between strength and toughness. However, when the V content exceeds 0.1% and becomes excessive, it causes a decrease in toughness due to an increase in strength. The upper limit with preferable V content is 0.08% or less (preferably 0.05% or less). In addition, the minimum with preferable V content for exhibiting the said effect is 0.005% or more (more preferably 0.01% or more).

[Nb: 0.005 to 0.05%]
Nb, like Cr, is an element necessary for suppressing the formation of ferrite and ensuring a balance between strength and toughness. In order to exert such effects, the Nb content needs to be 0.005% or more. However, when the Nb content exceeds 0.05% and becomes excessive, the toughness is reduced due to the increase in strength. The minimum with preferable Nb content is 0.007% or more (more preferably 0.01% or more), and a preferable upper limit is 0.040% or less (more preferably 0.035% or less).

[B: 0.005% or less (excluding 0%)]
B, like Cr, is an element necessary for suppressing the formation of ferrite and ensuring a balance between strength and toughness. However, if the B content exceeds 0.005% and becomes excessive, it causes a decrease in toughness due to an increase in strength. The upper limit with preferable B content is 0.004% or less (more preferably 0.003% or less). In addition, the minimum with preferable B content for exhibiting the said effect is 0.0005% or more (more preferably 0.001% or more).

[Ti: 0.005 to 0.02%]
Ti is an element effective for forming Ti nitride and improving toughness. In order to exhibit such an effect effectively, the Ti content needs to be 0.005% or more. However, if the Ti content is excessive, coarse nitrides are formed and the toughness is reduced, so it is necessary to make it 0.02% or less. The preferable lower limit of the Ti content is 0.006% or more (more preferably 0.008% or more), and the preferable upper limit is 0.015% or less (more preferably 0.01% or less).

[Al: 0.2% or less (excluding 0%)]
Al is useful as an element for fixing solute N that was not fixed by Ti. However, when the Al content exceeds 0.2% and becomes excessive, the toughness decreases. The preferable lower limit of the Al content is 0.010% or more (more preferably 0.020% or more), and the preferable upper limit is 0.100% or less (more preferably 0.070% or less).

[Ca: 0.0035% or less (excluding 0%)]
Ca is an element that makes MnS harmless and is effective in improving toughness due to the fine dispersion of MnS. However, if the Ca content is excessive, the toughness is reduced instead, so 0.0035% or less is necessary. Although this reason is not certain, it is considered to form coarse inclusions. A preferable lower limit for effectively exhibiting such an effect is 0.0005% or more (more preferably 0.001% or more), and a preferable upper limit is 0.003% or less (more preferably 0.0025%). The following).

[N: 0.003-0.006%]
N is an element that contributes to improving toughness by forming Ti nitride together with Ti. In order to effectively exhibit such an action, the N content needs to be 0.003% or more. However, when the N content is excessive, the amount of the solid solution Nb and the solid solution B is decreased, and the effect of Nb and B is not exhibited. The preferable lower limit of the N content is 0.0035% or more (more preferably 0.0040% or more), and the preferable upper limit is 0.0055% or less (more preferably 0.0050% or less).

  The contained elements defined in the present invention are as described above, and the balance is iron and inevitable impurities, and elements that are brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. (for example, H, As, O, etc.) Etc.) can be allowed to be mixed. Moreover, the thick steel plate of this invention may contain the following elements as needed, and the characteristic of a thick steel plate is further improved according to the kind by containing these.

[Mo: 0.5% or less (excluding 0%)]
Mo, like Cr, is an element useful for suppressing the formation of ferrite and ensuring a balance between strength and toughness. However, if the Mo content exceeds 0.5% and becomes excessive, it causes a decrease in toughness due to an increase in strength. The upper limit with preferable Mo content is 0.45% or less (more preferably 0.40% or less). In addition, the minimum with preferable Mo content for exhibiting the said effect is 0.05% or more (more preferably 0.1% or more).

[Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (not including 0%)]
Ni and Cu are both effective elements for increasing the strength of steel materials. However, if contained excessively, the strength is excessively increased and the toughness is adversely affected. Further, from the viewpoint of cost, it is preferable to contain it as much as possible. From these viewpoints, it is preferable that both be 0.5% or less. More preferably, both are 0.4% or less. In addition, the preferable minimum for exhibiting said effect effectively is all 0.05% or more (more preferably 0.1% or more).

  In order to manufacture the thick steel plate of the present invention, a steel material (steel material after casting) satisfying the chemical composition as described above is heated to a temperature range of 1000 to 1300 ° C., and is reduced at a reduction rate of 10% or more. (BD treatment) and then cooled to 600 ° C. or lower, and then reheated to a temperature range of 920 to 1200 ° C. to start hot rolling, and finished hot rolling at a finish rolling temperature of 700 ° C. or higher. It is necessary to cool to a bainite transformation start temperature or lower at a cooling rate of -20 ° C / second. The reason for setting the range of each condition in this manufacturing method is as follows.

[Heating temperature in BD treatment: 1000 to 1300 ° C.]
In order to sufficiently dissolve Nb (0.005% or more) or B (0.005% or less) in steel and to fully exhibit the effect of BD treatment (former γ homogenization effect), the heating temperature is 1000 ° C. It is necessary to do it above. However, if the heating temperature exceeds 1300 ° C., the initial austenite structure becomes too coarse, and it becomes difficult to sufficiently refine the austenite structure even if the austenite structure is rolled and recrystallized. A fine old γ structure cannot be obtained.

[Draft rate in BD treatment: 10% or more]
By setting the reduction ratio in the BD treatment to 10% or more (more preferably 20% or more) (by introducing strain), the structure before the hot rolling can be uniformly refined, and the subsequent hot rolling can be performed. However, some of the crystal grains are prevented from becoming coarse (the maximum equivalent circle diameter of the old γ grains is 150 μm or less), and a steel sheet with small variation in toughness is obtained. The upper limit of the rolling reduction at this time is preferably 50% or less (more preferably 40% or less) from the viewpoint of securing a predetermined rolling reduction in the subsequent rolling process. In addition, the said rolling reduction means the quantity represented by following (1) Formula, and when rolling is performed in multiple times within said temperature range, it means the total quantity (cumulative rolling reduction) (rolling) The same applies to the rolling reduction at the time).
Reduction ratio = (h1−h2) / h1 × 100 (%) (1)
However, h1: thickness before reduction, h2: thickness after reduction

[Cool to 600 ° C or less after BD treatment]
After the BD treatment, it is necessary to cool to 600 ° C. or lower from the viewpoint of using a phase transformation from γ to a low temperature transformation phase to refine the structure before rolling. When this temperature is higher than 600 ° C., the phase transformation cannot be fully utilized, and the structure before rolling is not sufficiently fine. As for cooling, cooling is performed so as not to add management items, but the cooling rate is not particularly limited.

[Heating temperature in rolling and rough rolling temperature: 920 to 1200 ° C., finish rolling temperature: 700 ° C. or higher]
In order to refine the structure after transformation (bainite structure), it is effective to roll and recrystallize the austenite structure. And the recrystallization of austenite (minimum temperature at which recrystallization starts) depends on the chemical composition of the steel material, but is usually 920 ° C. or higher (rough rolling temperature) for the chemical composition defined in the present invention. ). In order to perform rolling in such a temperature range, the heating temperature of the steel material needs to be 920 ° C. or higher (preferably 950 ° C. or higher). However, if the heating temperature is too high, the austenite structure before rolling itself becomes coarse and the structure after transformation cannot be refined, so that it is necessary to set it to 1200 ° C. or less (preferably 1180 ° C. or less). Further, the finish rolling temperature is preferably 900 ° C. or lower, but if it is lower than 700 ° C., ferrite formation is promoted and the amount of bainite tends to be insufficient.

[Cooling rate after hot rolling: 1 to 20 ° C./second]
About the cooling rate after hot rolling, in order to transform in the flat part of a CCT curve (continuous cooling transformation curve), it is necessary to make the cooling rate into the range of 1-20 degreeC / sec. When this average cooling rate is less than 1 ° C./second, not only productivity is lowered, but also ferrite formation is promoted. On the other hand, when the cooling rate exceeds 20 ° C./second, the formation of a martensite structure is promoted, and the strength of the steel sheet increases excessively. The preferable lower limit of the cooling rate is 2 ° C./second or more (more preferably 3 ° C./second or more), and the preferable upper limit is 15 ° C./second or less (more preferably 12 ° C./second or less). The cooling stop temperature needs to be cooled to at least the bainite transformation start temperature or lower (for example, 450 ° C. or lower), but may be cooled to room temperature.

A thick steel plate with the structure and old γ grains adjusted appropriately can be obtained by the manufacturing method as described above. However, if necessary, a tempering treatment can be performed in a temperature range of 500 ° C. or higher and an Ac ₁ transformation point or lower. By performing such a treatment, the movable dislocations introduced during hot rolling and cooling are reduced, and the yield point YP can be stabilized (variation reduced).

  The present invention relates to a thick steel plate. In this field, a thick steel plate generally refers to one having a plate thickness of 3.0 mm or more as defined by JIS. However, the thickness of the thick steel plate targeted in the present invention is preferably about 60 mm or more, more preferably about 80 mm or more and 100 mm or less. That is, in the present invention, even a steel plate having a large thickness exhibits good toughness and high strength. The steel plate of the present invention thus obtained can be used as a material for structures such as bridges, high-rise buildings, ships and tanks.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

  After melting with a steel material having the chemical composition shown in Table 1 below and casting to form a slab, the conditions shown in Table 2 below (the presence or absence of BD treatment, heating temperature before rough rolling, total reduction during rough rolling) Rate, finish rolling temperature, finish thickness, cooling rate after rolling, and tempering temperature) to produce various high-strength thick steel plates. In addition, the BD process at this time heated up to 1250 degreeC and performed reduction by the reduction rate of 10%.

About each obtained steel plate, according to the following procedures, structure (bainite fraction, average equivalent circle diameter of old γ grains, maximum equivalent circle diameter of old γ grains), steel plate strength (yield point YP, tensile strength TS), toughness [Average value (vE _-5 ave) and minimum value (vE _-5 min) of Charpy impact absorption energy at -5 ° C] was measured.

[Bainite fraction]
A test piece obtained by mirror-polishing a cross section parallel to the rolling direction at the t / 4 position (t: plate thickness) of each steel plate was etched with a 2% nital solution, and an observation field of view: a range of 200 μm × 150 μm was measured with an optical microscope. Using this, 10 fields of view were photographed at 400 times. About these 10 visual fields, the image analysis was performed using "Image-Pro Plus" by Media Cybernetics, and the bainite fraction (area%) was measured. At this time, a lath-like structure other than a mixed structure (MA structure) composed of ferrite, pearlite, martensite and austenite was regarded as bainite.

[Average circle equivalent diameter of old γ grains, maximum equivalent circle diameter of old γ grains]
A test piece obtained by mirror-polishing a cross section parallel to the rolling direction at the t / 4 position (t: plate thickness) of each steel plate was etched with a 2% nital solution, and an observation field of view: a range of 200 μm × 150 μm was measured with an optical microscope. Using this, 5 fields of view were photographed at 100 times. These five visual fields were subjected to image analysis using “Image-Pro Plus” manufactured by Media Cybernetics, and the average equivalent circle diameter and the maximum equivalent circle diameter of the old γ grains in the tissue were measured.

[Measurement of steel plate strength]
JIS Z22014 test specimens were sampled at right angles to the rolling direction from t / 4 position (t: thickness) of each steel plate obtained, and subjected to a tensile test according to JIS Z2241 (each 3 times), yield point YP and tensile strength. The strength TS was measured. A material having a yield point YP ≧ 500 MPa and a tensile strength TS of 570 to 720 MPa was evaluated as having excellent steel plate strength.

[Measurement of toughness (Charpy impact absorption energy)]
Three Charpy impact test pieces (JIS Z2201 No. 4 test piece) were sampled from t / 4 positions (t: thickness) of each steel plate obtained, and Charpy impact absorption energy (vE at -5 ° C) was collected. _-5 ) was measured, and those having an average value (vE _-5 ave) and a minimum value (vE _-5 min) of 200 J or more were evaluated as having excellent toughness.

  The measurement results are shown in Table 3 below.

From this result, it can be considered as follows. Test No. Nos. 1 to 15 satisfy the requirements of the present invention for both the component composition and the production conditions. Therefore, a thick steel plate having excellent toughness (vE _-5 ave and vE _-5 min) as well as strength (yield point YP and tensile strength TS) is obtained. Has been obtained.

  On the other hand, test no. 16 to 24 are examples in which at least one of the component composition and production conditions did not satisfy the requirements of the present invention.

Test No. Since No. 16 has a large Cr content, the strength (tensile strength TS) is too high, and the toughness (vE _-5 ave and vE _-5 min) is deteriorated. Test No. In No. 17, the Cr content is low, ferrite transformation cannot be suppressed (the bainite fraction is low), and the steel sheet strength (yield point YP) is reduced.

Test No. No. 18 has a high Mo content, the strength (tensile strength TS) becomes too high, and the toughness (vE _-5 ave and vE _-5 min) is deteriorated. Test No. In No. 19, the V content is increased, the strength (tensile strength TS) is too high, and the toughness (vE _-5 ave and vE _-5 min) is deteriorated.

Test No. In No. 20, the Nb content is increased, the strength (tensile strength TS) is too high, and the toughness (vE _-5 ave and vE _-5 min) is deteriorated. Test No. No. 21 has a low Nb content, cannot suppress ferrite transformation (low bainite fraction), and has reduced steel plate strength (yield point YP and tensile strength TS).

Test No. In No. 22, the B content is increased, the strength (tensile strength TS) is too high, and the toughness (vE _-5 ave and vE _-5 min) is deteriorated. Test No. No. 23 is an example in which the BD treatment was not performed before hot rolling using the steel type A, and the maximum equivalent circle diameter of the old γ grains was large, and the minimum toughness (vE ₋₅ min) was 200 J. The above has not been secured.

Test No. 24 is an example in which the rough rolling temperature is set to 900 ° C. using the steel type A, and the average equivalent circle diameter and the maximum equivalent circle diameter of the old γ grains are increased, and the toughness (vE ₋₅ ave and vE ₋₅ is increased). min) has deteriorated.

Claims (3)

C: 0.01 to 0.07% (meaning mass%, hereinafter the same for the chemical component composition),
Si: 0.5% or less (excluding 0%),
Mn: 1 to 1.7%,
P: 0.015% or less (excluding 0%),
S: 0.006% or less (excluding 0%),
Cr: 0.8-2%,
V: 0.005-0.1 %,
Nb: 0.005 to 0.05%,
B: 0.0005 to 0.005 %,
Ti: 0.005 to 0.02%,
Al: 0.2% or less (excluding 0%),
Ca: 0.0035% or less (excluding 0%), and N: 0.003-0.006%,
Each of which is iron and inevitable impurities,
More than 90 area% of the steel structure is bainite,
In addition, a high strength thick steel plate having excellent toughness, wherein the average equivalent circle diameter of the prior austenite grains is 5 μm or more and 95 μm or less, and the maximum equivalent circle diameter of the prior austenite grains is 150 μm or less.   The high-strength thick steel plate according to claim 1, further comprising Mo: 0.5% or less (not including 0%).   The high-strength thick steel plate according to claim 1 or 2, further comprising Ni: 0.5% or less (excluding 0%) and / or Cu: 0.5% or less (excluding 0%).