JP5139015B2 - Thick high-strength steel sheet for large heat input welding with low base metal low-temperature toughness variation and excellent heat-affected zone toughness, and method for producing the same - Google Patents

Description

  The present invention relates to a thick-walled high-strength steel sheet that is excellent in low-temperature toughness of a base material and also excellent in toughness of a weld heat-affected zone when subjected to high heat input welding, and a method for producing the same.

  When welding a thick steel plate (thick material) having a thickness of about 60 to 80 mm, large heat input welding is generally applied from the viewpoint of improving welding construction efficiency. However, in such high heat input welding, the heat input of welding reaches 100 to 500 kJ / cm, and the toughness of the heat-affected zone (hereinafter sometimes abbreviated as “HAZ”) of the base material (steel plate) is likely to deteriorate. Ensuring such toughness is an important requirement. Various techniques for improving the HAZ toughness have been proposed so far.

  In the steel sheet as described above, not only the toughness of the HAZ but also the base material itself has high strength and high toughness. Regarding such characteristics, in a thin steel plate (thin material) having a thickness of less than 60 mm, high toughness is stably ensured without “variation” because of easy structure control. On the other hand, since it is difficult to introduce distortion due to rolling in the above-described thick materials, it is actually difficult to ensure high toughness stably without “variation”.

In order to improve the toughness of the base material, particularly the toughness at low temperatures, it is known that refinement of the structure is effective. For example, Patent Document 1 discloses that after hot rolling at a finish rolling temperature of not less than Ar ₃ transformation point and not more than 900 ° C., control of recovery of the structure of the steel sheet or improvement of hardenability by B in a solid solution state. In addition, there has been proposed a technique for achieving fine structure by starting accelerated cooling within 20 seconds after the end of rolling.

With the above-described technology, high HAZ toughness is realized with a steel plate having a thickness of about 20 to 80 mm. However, the technology includes a thick steel plate with a thickness of 60 mm or more. For such a thick steel plate, the microstructure is refined by rolling, which is the most effective in improving the toughness of the base material but is difficult to control. However, since the microstructure of the base material is refined by control by accelerated cooling, the desired high toughness cannot be stably ensured, and the toughness level is -20 ° C. The fact is that we can only compensate.
Japanese Patent No. 3899014

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to stably ensure excellent low temperature toughness of the base material without variation, and also to affect the heat of welding when performing high heat input welding. An object of the present invention is to provide a thick high-strength steel plate excellent in toughness of the part and a useful method for producing such a steel plate.

The thick-walled high-strength steel sheet of the present invention that has achieved the above object is C: 0.03 to 0.08% (meaning mass%, the same shall apply hereinafter), Si: 0.05% or less (0% Mn: 1.4 to 1.7%, Al: 0.01 to 0.06%, P: 0.05% or less (excluding 0%), S: 0.01% or less (0% Cu: 0.20-0.40%, Ni: 0.20-0.60%, Nb: 0.005-0.015%, Ti: 0.005-0.03%, B : 0.0005-0.0030% and N: 0.0030-0.0090%, respectively, the balance consists of iron and inevitable impurities, and consists of a structure mainly composed of bainite phase,
When a region surrounded by a large-angle grain boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as a crystal grain at a position of depth t / 4 from the surface (t represents a plate thickness, the same applies hereinafter), the maximum The main point is that the crystal grain size is 20 μm or less in terms of the equivalent circle diameter.

  In the present invention, “mainly composed of a bainite phase” means a state in which the bainite phase occupies 90 area% or more in the structure. The “equivalent circle diameter” means a diameter (equivalent circle diameter) when converted to a circle having the same area.

  In the thick high-strength steel sheet of the present invention, if necessary, (1) Cr: 0.20% or less (not including 0%), Mo: 0.10% or less (not including 0%), and V: 0 It is also effective to contain at least one selected from the group consisting of 0.040% or less (not including 0%), (2) Ca: 0.004% or less (not including 0%), etc. The properties of the steel sheet are further improved depending on the components to be contained.

In the thick high-strength steel sheet of the present invention as described above, high base metal low temperature toughness such that the minimum value of Charpy absorbed energy vE- ₄₀ at -40 ° C is 150 J or more can be stably secured.

  On the other hand, in manufacturing the thick high-strength steel sheet of the present invention as described above, when the steel slab is heated to a temperature of 950 to 1200 ° C. and subjected to hot rolling, the depth t / 4 from the steel sheet surface. In the range where the temperature at 830 ° C. or more and 860 ° C. or less, rolling reduction: 5% or more is performed, and in the range where the temperature is 770 ° C. or more and less than 810 ° C., the reduction rate: 5% or more. And rolling at a temperature of 810 ° C. or more and less than 830 ° C. is 2% or less (including 0%).

  In the present invention, in a steel sheet having a structure mainly composed of a bainite phase, the chemical composition is strictly defined, and a region surrounded by a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more is defined as a crystal grain. In this case, by refining the largest crystal grains, the excellent low-temperature toughness of the base metal can be ensured stably without variation, and the toughness of the heat affected zone when welding with high heat input is also excellent. Thick, high-strength steel sheets can be realized, and such steel sheets are useful as materials for various large welded structures including shipbuilding and bridge fields.

  In order to solve the above-mentioned problems, the present inventor has focused particularly on a steel sheet having a bainite structure, and has excellent base metal strength and low-temperature toughness in the steel sheet, and also excellent in HAZ toughness when subjected to high heat input welding. We studied from various angles in order to realize this. As a result, the following knowledge was obtained. In the past, it was thought that the base material toughness (Charpy impact absorption characteristics) would be improved by refining the average value of crystal grains. Coarse crystal grains may be present, and it has been found that the presence of such coarse crystal grains causes a decrease in toughness and variations thereof, particularly in thick steel plates.

  The present inventor paid attention to such a phenomenon, and proceeded with studies from the viewpoint of eliminating the presence of coarse crystal grains as much as possible. As a result, when using steel plates with strictly defined chemical composition, manufactured under appropriate conditions, the region surrounded by large-angle grain boundaries where the orientation difference between adjacent crystals is 15 ° or more is used as the crystal grains. If the maximum crystal grains are made finer, the presence of coarse crystal grains can be avoided, and a high-thickness high-strength steel sheet with excellent HAZ toughness can be realized while avoiding toughness deterioration and variations. The present invention has been completed.

  In the bainite structure, it forms with some orientation relation to austenite, but the orientation of each crystal lattice selected by the chemical composition of the steel sheet, the formation temperature of the structure, other conditions, etc. The relationship has changed, and it has been found that the low temperature toughness of the base material is good at the grain boundaries having a certain crystal orientation difference. If the maximum crystal grain is properly defined, good low temperature toughness of the base material can be realized without causing variation in characteristics due to the presence of coarse crystal grains.

  In a single-phase structure mainly composed of bainite phase, it is considered that the grain boundary acts as a resistance to crack growth, but if the frequency of the collision between the grain boundary and the crack is increased during crack growth, the crack growth is suppressed. This is considered to improve the toughness of the base material. However, in a small-angle grain boundary (small tilt boundary) where the orientation difference between both ends forming the grain boundary is small (for example, less than 15 °), the grain boundary energy is small and the effect is small, so the orientation difference is 15 °. It is necessary to target the above large-angle grain boundaries (large tilt boundaries).

  In other words, the maximum value of the diameter (equivalent circle diameter) when converted to a circle of the same area with a crystal grain surrounded by a large-angle grain boundary having a misorientation of 15 ° or more at a depth t / 4 from the surface. By setting the (maximum grain boundary diameter) to 20 μm or less, a thick high-strength steel sheet suitable for the above purpose could be realized. In the steel sheet of the present invention, in order to improve the base material characteristics, the crystal grain orientation relationship was evaluated at the position of the depth t / 4 from the surface, which is a representative position of the entire plate thickness, and the low temperature toughness is This is because it needs to be suppressed at the full thickness.

  The “orientation difference” is also referred to as “shift angle” or “inclination angle”, and may be hereinafter referred to as “crystal orientation difference”. In addition, the measurement of the crystal orientation difference can be realized by employing the above-described electron backscattering diffraction image method (Electron Backscattering Pattern method: hereinafter referred to as “EBSP method”).

  Moreover, in a thick high-strength steel sheet, high strength (for example, tensile strength TS: 490 MPa or more) can be realized by using a structure mainly composed of bainite.

  One feature of the steel sheet of the present invention is that the chemical composition is appropriately adjusted. Below, the reason for limiting the range of chemical components will be described.

[C: 0.03-0.08%]
C is an element necessary for ensuring the strength of the steel sheet. In order to obtain a high strength, that is, a tensile strength TS of about 490 MPa (depending on the thickness of the steel sheet used), it is necessary to contain 0.03% or more. However, if the content exceeds 0.08%, weldability deteriorates. For these reasons, the C content is set to 0.03 to 0.08%. In addition, the preferable minimum of C content is 0.04%, and a preferable upper limit is 0.06%.

[Si: 0.05% or less (including 0%)]
Since Si is an element that promotes austenitization in HAZ of high heat input welding, it is necessary to make it 0.05% or less. Preferably it is 0.03% or less.

[Mn: 1.4 to 1.7%]
Mn is an effective element for securing the strength and toughness of the steel sheet, and in order to exert such effects, it is necessary to contain 1.4% or more. However, if it is contained excessively, weldability and cracking susceptibility deteriorate, so it is necessary to make it 1.7% or less. In addition, the minimum with preferable Mn content is 1.5%, and a preferable upper limit is 1.6%.

[Al: 0.01 to 0.06%]
Al is an element useful for deoxidation, and if less than 0.01%, there is no deoxidation effect. However, if it is contained excessively, the toughness of the welded portion is deteriorated, so it is necessary to make it 0.06% or less.

[P: 0.05% or less (excluding 0%)]
P is an impurity that segregates in crystal grains and adversely affects ductility and toughness. Therefore, it is preferable that P be as small as possible, but it should be suppressed to 0.05% or less in consideration of the degree of cleanliness of practical steel. Is good. In addition, P is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[S: 0.01% or less (excluding 0%)]
S is an impurity that combines with alloy elements in the steel sheet to form various inclusions and adversely affects the ductility and toughness of the steel sheet, so it is preferable that it be as small as possible. Considering the degree, it is preferable to suppress it to 0.01% or less. In addition, S is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[Cu: 0.20 to 0.40%]
Cu is effective in forming fine blocks by suppressing transformation and lowering the bainite transformation point Bs. In order to exhibit such an effect, it is necessary to contain Cu 0.20% or more. However, since the weldability is impaired when the amount is excessive, the upper limit needs to be 0.40%. In addition, the minimum with preferable Cu content is 0.25%, and a preferable upper limit is 0.35%.

[Ni: 0.20 to 0.60%]
Ni, like Cu, is effective in forming fine blocks by suppressing transformation and lowering the bainite transformation point Bs. In order to exert such effects, Ni needs to be contained by 0.20% or more. However, since the weldability is impaired when the amount is excessive, the upper limit needs to be 0.60%. In addition, the minimum with preferable Ni content is 0.30%, and a preferable upper limit is 0.40%.

[Nb: 0.005 to 0.015%]
Since Nb has the effect of suppressing recrystallization of austenite during rolling, the austenite grains can be refined and the structure after transformation can be refined. In order to exert such effects, it is necessary to contain Nb in an amount of 0.005% or more (preferably 0.006% or more). However, since an excessive content impairs weldability, the Nb content is preferably 0.015% or less (preferably 0.012% or less).

[Ti: 0.005 to 0.03%]
Ti precipitates during high heat input welding and exhibits a pinning effect that suppresses austenite grain coarsening of HAZ. In order to exert such effects, it is necessary to contain Ti in an amount of 0.005% or more (preferably 0.007% or more). However, if the Ti content is excessive, weldability is impaired, so the content was made 0.03% or less (preferably 0.025% or less).

[B: 0.0005 to 0.0030%]
B is effective for forming fine blocks by suppressing transformation and reducing Bs. In order to exert such effects, it is necessary to contain 0.0005% or more. However, if the B content is excessive, weldability is impaired, so the content was made 0.0030% or less.

[N: 0.0030 to 0.0090%]
N is an element that improves the HAZ toughness by forming a nitride with an element such as Ti or Al. In order to exert such an effect, N needs to be contained in an amount of 0.0030% or more (preferably 0.0040% or more). In addition, the solid solution N causes the HAZ toughness to deteriorate. With the increase in the total nitrogen amount, the aforementioned nitride increases, but the solid solution N also becomes excessive. Therefore, in the present invention, it is suppressed to 0.0090% or less.

  The basic components in the steel sheet of the present invention are as described above, and the balance consists of iron and inevitable impurities (for example, O). In addition to the above components, it is also effective to add the following components to the steel sheet of the present invention as necessary.

[Selected from the group consisting of Cr: 0.20% or less (not including 0%), Mo: 0.10% or less (not including 0%), and V: 0.040% or less (not including 0%) One or more types]
Like these Cu and Ni, these elements are effective for forming fine blocks by suppressing transformation and reducing Bs, and are contained as necessary. When these elements are contained, the effect increases as the content increases, but when the amount is excessive, weldability is impaired. Therefore, the upper limit in the case of containing these elements is determined as described above.

[Ca: 0.004% or less (excluding 0%)]
Ca is an element effective for improving the toughness by the fixation of S, and in order to exert the effect, it is preferable to contain 0.001% or more. However, since the effect is saturated even if it contains excessively, it is preferable to make it 0.004% or less.

  The high-strength steel sheet of the present invention can achieve the above effects by appropriately controlling its chemical composition and defining its structure and crystal grains (maximum crystal grain diameter). What is necessary is just to manufacture according to the following method in order to implement | achieve a high strength steel plate.

  First, a steel slab that satisfies the above-mentioned chemical composition composition is heated to a temperature range of 950 to 1200 ° C. (the reference position is a position at a depth t / 4 from the steel slab surface), and then hot-rolled. When performing rolling in a temperature range of 770 ° C. or higher and lower than 810 ° C., which will be described later, it is necessary to heat to a temperature range of 950 to 1200 ° C. in order to ensure a predetermined rolling reduction. Further, if the heating temperature is less than 950 ° C., Nb contained in the steel slab does not dissolve, and if it exceeds 1200 ° C., the austenite grains become coarse, so even if effective rolling is performed, the structure Therefore, it is impossible to achieve finer toughness and stable high toughness.

  As specific conditions for hot rolling, the following three stages of rolling may be performed. First, in the range where the temperature from the steel piece surface (steel plate surface) to a position at a depth t / 4 (hereinafter sometimes simply referred to as “t / 4 part”) is 830 ° C. or higher and 860 ° C. or lower, the rolling reduction ratio: It is necessary to perform rolling of 5% or more. The effective recrystallization region for the above-mentioned austenite grain refinement of the component system is 830 to 860 ° C. By carrying out rolling at a reduction rate (cumulative reduction rate) of 5% or more in this temperature range, fine austenite is obtained. It can be made into a structure, and can finally be made into a fine transformed structure (after transformation).

  On the other hand, if the reduction ratio in the above temperature range is less than 5%, the reduction in the recrystallization region at the t / 4 part becomes insufficient, and coarse austenite grains are mixed during rolling. When the structure is in such a state, variations in low-temperature toughness due to a coarse structure tend to occur.

The rolling reduction is a value (cumulative rolling reduction) calculated from the following formula (1) (the same applies to rolling reduction in rolling described later).
Reduction ratio = (t ₀ −t ₁ ) / t ₂ × 100 (1)
[In formula (1), t ₀ is the rolling start thickness (mm) of the steel slab when the temperature of the t / 4 part of the steel slab is within the rolling temperature range, and t ₁ is the temperature at the t / 4 position of the steel slab. there completion of rolling the thickness of the steel strip when in the rolling temperature range (mm), t ₂ is the thickness of the pre-rolling the slab, representing respectively. ]

  After performing the above rolling, rolling is performed at a rolling reduction of 2% or less (including 0%) when the temperature of the t / 4 part of the steel slab is 810 ° C. or more and less than 830 ° C. In the steel sheet of the present invention, a uniform fine structure is achieved by the reduction in the above-mentioned recrystallization region and the non-recrystallization region described later, whereby the Charpy absorbed energy at −40 ° C. is 150 J or more. , To achieve excellent low temperature toughness stably. However, the temperature range of 810 ° C. or more and less than 830 ° C. is a temperature range in which a recrystallized portion and an unrecrystallized portion are mixed (austenite partially non-recrystallized temperature range). In some cases, recrystallized grains take in unrecrystallized grains and grow into huge grains in order to reduce distortion of the unrecrystallized grains. Therefore, it is necessary to avoid rolling as much as possible in this temperature range. However, it is permissible to perform rolling to the extent that the distortion of rolling at the t / 4 portion, which is the object of toughness evaluation, is not substantially affected (the rolling reduction is about 2% or less).

  As final rolling conditions, it is necessary to perform rolling with a reduction ratio of 5% or more in a temperature range of 770 ° C. or higher and lower than 810 ° C. In the steel sheet of the present invention, the non-recrystallized region effective for introducing strain into austenite grains in the above component system is a temperature range of 770 ° C. or more and less than 810 ° C. By carrying out rolling such that the rolling reduction (cumulative rolling reduction) is 5% or more in this temperature range, the austenite grains refined by the strain introduced by rolling are made a finer structure after transformation. Can do. If the rolling reduction at this time is less than 5%, sufficient strain introduction necessary for miniaturization after transformation at the t / 4 portion cannot be realized. Also, if the rolling temperature at this time is less than 770 ° C., ferrite begins to precipitate partially during cooling after rolling, so the strength improvement effect due to transformation is reduced, and the strength becomes insufficient and high strength is difficult to be achieved. .

  After rolling as described above, in order to ensure the strength of the steel sheet, it is necessary to cool at least at 750 ° C. or more in order to ensure that the t / 4 part has a bainite structure (structure mainly composed of bainite phase). It is necessary to start and cool at a cooling rate of 2 ° C./second or more (for example, water cooling). Note that the cooling stop temperature at this time needs to be 450 ° C. or lower in order to obtain a structure mainly composed of a bainite phase.

  By the manufacturing method as described above, it is possible to manufacture a thick high-strength steel sheet that satisfies the requirements of the chemical component composition and the structural requirements of the present invention and does not vary in low-temperature toughness. The thickness is preferably about 60 to 80 mm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
Steels (steel types A to U) having the chemical composition shown in Table 1 below were melted in a converter and hot-rolled under the conditions shown in Table 2 below to produce various steel plates. The temperature at t / 4 part of the steel slab was calculated by a process computer using a difference method. The specific temperature management procedure is as follows. Further, the Ar ₃ transformation point shown in the following Table 1 adopts a value obtained by the following equation (2).

Ar ₃ transformation point (° C.) = 868-369. [C] +24.6. [Si] -68.1. [Mn] -36.1. [Ni] -20.7. [Cu] -24.8 [Cr] +29.6 [Mo] (2)
However, [C], [Si], [Mn], [Ni], [Cu], [Cr] and [Mo] are the contents (mass of C, Si, Mn, Ni, Cu, Cr and Mo, respectively). %).

[Temperature measurement method during rolling]
1. In the process computer, the heating temperature at a predetermined position (t / 4 part from the surface) of the billet is calculated based on the atmospheric temperature from the start of heating to extraction and the in-furnace time.
2. Using the calculated heating temperature, based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between passes, a method suitable for calculation such as the difference method is used to calculate the rolling temperature at any position in the plate thickness direction. The rolling is carried out while using the calculation.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5. If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, recalculate the calculated surface temperature so that it matches the measured temperature to obtain the calculated temperature on the process computer. The calculated temperature is used as it is.
6). Using the calculated temperature calculated above, the rolling temperature in the region to be controlled is managed.

  About each obtained steel plate, a bainite fraction, a large angle grain boundary diameter (maximum equivalent circle diameter), a base material tensile characteristic, a base material toughness (base metal impact characteristic), and HAZ toughness (large heat input characteristic) are as follows. Measured (both t / 4 parts).

[Measurement of bainite fraction]
A sample was cut out from a depth t / 4 part of the steel plate so that a plane parallel to the rolling direction of the steel plate and perpendicular to the surface of the steel plate was exposed, and this was used as wet emery polishing paper from # 150 to # 1000 And then mirror finished using a diamond slurry as an abrasive. After this mirror-polished piece was etched with a 2% nitric acid-ethanol solution (a nital solution), a 150 μm × 200 μm field of view was observed at an observation magnification of 400 times, and the bainite fraction was measured by image analysis. All lath structures other than ferrite were regarded as bainite. The ferrite fraction of 5 fields of view was calculated in total and the average value was adopted.

[Measurement of large-angle grain boundary diameter (maximum equivalent circle diameter)]
In a cross section parallel to the rolling of the steel plate, the large-angle grain boundary diameter was measured by FE-SEM-EBSP (electron backscatter diffraction image method using an electron emission scanning electron microscope). Specifically, an EBSP apparatus (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with an FE-SEM, and a boundary having an inclination (crystal orientation difference) of 15 ° or more is used as a grain boundary. The field diameter was measured. The measurement conditions at this time are: measurement area: 200 μm × 200 μm, measurement step: 0.5 μm interval, and measurement points whose confidence index (Confidence Index) indicating the reliability of the measurement direction is smaller than 0.1 are analyzed. Excluded. The maximum value of the large-angle grain boundary diameter thus obtained was calculated and used as the large-angle grain boundary diameter (maximum equivalent circle diameter) in the present invention. In addition, about the thing with a crystal grain diameter of 2.0 micrometers or less, it was judged as measurement noise and was excluded from the object of calculation.

[Tensile properties of base material]
JIS Z 2201 No. 4 test piece was taken from t / 4 part of each steel plate in the direction perpendicular to the rolling direction, and the tensile strength TS was measured by conducting a tensile test according to JIS Z 2241. The standard of the tensile strength TS was 490 MPa or more.

[Evaluation of low temperature toughness of base metal]
A JIS Z 2242 test piece was taken from the t / 4 part of the steel plate in a direction parallel to the rolling direction, and a Charpy impact test was performed according to JIS Z 2242. Test temperature: Three impact tests were conducted at -40 ° C, and the minimum absorbed energy vE- ₄₀ was determined to be 150 J or more.

[Evaluation of high heat input HAZ toughness]
The butt groove is welded by electrogas welding (EGW) (the amount of heat input is shown in Table 4 below), and a JIS Z 2242 test piece is taken from the root side (back side) and placed at the bond part or (bond part + 1 mm) position. The test was carried out according to JIS Z 2242 with notches. Test temperature: Three impact tests were conducted at −20 ° C., and the absorbed energy vE- ₂₀ was 100 J or more in all cases.

These results are shown in the following Tables 3 and 4 (Table 3 is base material characteristics, Table 4 is HAZ toughness and welding conditions), and these results can be considered as follows. First, experiment no . Those of 2 to 4 , 6 to 10 , 12 to 14 , 18 , and 19 satisfy the requirements specified in the present invention, and good low temperature toughness is stable without variation in a state where the strength of the base material is high. It can be seen that the HAZ toughness is also good. On the other hand, those lacking any of the requirements defined in the present invention (Experiment No. 5 , 15-17 , 20-26 ) show that any of the characteristics is deteriorated. In particular, it can be seen that when the maximum equivalent circle diameter is large, the low temperature toughness of the base material varies.

Claims (4)

C: 0.03 to 0.08% (meaning mass%, the same shall apply hereinafter), Si: 0.05% or less (including 0%), Mn: 1.4 to 1.7%, Al: 0.01 -0.06%, P: 0.05% or less (not including 0%), S: 0.01% or less (not including 0%), Cu: 0.20 to 0.40%, Ni: 0 20 to 0.60%, Nb: 0.005 to 0.015%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0030%, Ca: 0.004% or less (0 %) And N: 0.0030 to 0.0090%, respectively, the balance consisting of iron and inevitable impurities, and consisting of a structure mainly composed of bainite phase,
When a region surrounded by a large-angle grain boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as a crystal grain at a position of depth t / 4 from the surface (t represents a plate thickness, the same applies hereinafter), the maximum A thick high-strength steel sheet for large heat input welding with less variation in the base metal low-temperature toughness and excellent toughness in the heat-affected zone, wherein the crystal grain size is 20 μm or less in terms of the equivalent circle diameter.   Furthermore, from the group consisting of Cr: 0.20% or less (not including 0%), Mo: 0.10% or less (not including 0%), and V: 0.040% or less (not including 0%) The thick high-strength steel sheet for high heat input welding according to claim 1, which contains one or more selected. The thick high-strength steel sheet for high heat input welding according to claim 1 or 2 , wherein the minimum value of Charpy absorbed energy vE- ₄₀ at -40 ° C is 150 J or more. In manufacturing the thick high-strength steel sheet for high heat input welding according to any one of claims 1 to 3 , the steel slab is heated to a temperature of 950 to 1200 ° C and subjected to hot rolling. In the range where the temperature at the position of the depth t / 4 from the surface is 830 ° C. or more and 860 ° C. or less, the rolling ratio is 5% or more, and the temperature is 770 ° C. or more and less than 810 ° C. , reduction ratio: for 5% or more of the rolling, and the temperature is 810 ° C. or higher, have rows rolling reduction ratio of less than 830 ° C. as a 2% or less (including 0%), then cooled from 750 ° C. or higher Starts and stops cooling at 450 ° C. or less with a cooling rate of 2 ° C./s or more, with low variation in the low temperature toughness of the base metal and high heat-affected zone toughness for high heat input welding A method for producing a strength steel plate.