JP5096087B2 - High tensile strength steel plate for high heat input welding with excellent base metal low temperature toughness - Google Patents

Description

  The present invention relates to a high-strength steel plate used as an extremely thick material (plate thickness: 50 to 80 mm) mainly used for ships, and particularly has excellent base material strength and toughness and welding heat when high heat input welding is performed. The present invention relates to a high-tensile steel plate for high heat input welding that is excellent in toughness of the affected part.

  In recent years, the size of major ship types has been increasing, especially in container ships, the number of stacks has increased from 6000 TEU to 10,000 TEU, which is the most severe ship type with the highest longitudinal strength at the large deck opening. The use of extra-thick material is necessary. In addition, when using high-strength steel sheets, toughness grades must be taken into consideration because brittle fracture is suppressed, and high strength steel sheets with high toughness grades are important components for securing longitudinal strength even in container ships. Will be needed.

  When assembling a hull block to which the above-described high strength / high toughness grade steel plate is applied, one-pass welding of standing electrogas is generally performed from the viewpoint of ensuring construction efficiency. Moreover, when performing such welding, the welding heat input amount ranges from 10 to 50 kJ / mm, and the toughness of the heat affected zone of the base material (steel plate) (hereinafter sometimes abbreviated as “HAZ”) is ensured. That is also an important requirement.

  As described above, steel plates used for ships are required to have good HAZ toughness in high heat input welding as well as high strength and high toughness. In addition, various techniques have been proposed so far for ensuring such required characteristics.

  For example, Patent Document 1 proposes that a steel plate contains a relatively large amount of Cu and Ni from the viewpoint of base material strength and base material toughness. However, since Cu and Ni are expensive alloy components, the cost of the steel sheet is significantly increased.

On the other hand, Patent Document 2 proposes a technique for achieving high strength / high toughness by making the structure ultrafine ferrite without adding expensive Ni. However, in order to make the microstructure ultrafine ferrite, it is necessary to go through at least one reverse transformation step in which cooling is stopped at a temperature equal to or lower than the Ar ₃ transformation point and double heating is performed, which is not preferable from the viewpoint of productivity.
JP 2006-2236 A Japanese Patent No. 3845113

  The present invention has been made by paying attention to the circumstances as described above, and its purpose is not to cause problems of cost and productivity, but excellent in the strength and toughness of the base material, and when high heat input welding is performed. An object of the present invention is to provide a high-strength steel plate for high heat input welding that is excellent in the toughness of the weld heat affected zone.

The high-strength steel sheet for high heat input welding of the present invention that has achieved the above object is C: 0.06 to 0.12% (meaning of mass%, the same shall apply hereinafter), Si: 0.05 to 0.00. 5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.06%, P: 0.025% or less (excluding 0%), S: 0.01% or less (0% Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009%, and B: 0.0005 to 0.003%, respectively. And the carbon equivalent Ceq defined by the following formula (1) is 0.40% or less, and the balance consists of iron and inevitable impurities,
It consists of an organization 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 (t represents the plate thickness, the same applies hereinafter) from the surface, the crystal The average equivalent circular diameter D _{A of the} grains measured by electron backscatter diffraction imaging is 10 μm or less,
The predicted maximum particle size D _M calculated by the extreme value statistical method based on the following formulas (2) to (7) is 80 μm or less for the crystal grain size measured by the electron backscatter diffraction image method. It has a gist.

Ceq (%) = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (1)
However, [C], [Mn], [Cr], [Mo], [V], [Cu] and [Ni] are the contents (mass of C, Mn, Cr, Mo, V, Cu and Ni, respectively). %).
D _M = ay + b (2)
y = −ln {−ln [(T−1) / T]} (3)
T = (A _s + A) / A (4)
A = A ₀ (5)
y (i) = aD (i) + b (6)
y (i) = − ln {−ln [i / (n + 1)]} (7)
Where a, b: slope and intercept of equation (6), y: normalization variable, T: recursion period,
A _s : prediction target area (mm ² ), A: inspection reference area (mm ² ),
A ₀ : measurement area (mm ² ) of electron backscatter diffraction image method, n: number of observation fields,
D (i): crystal grain size measured by electron backscatter diffraction image method, showing the maximum size at A ₀ and i: observation field number.

  If necessary, the steel sheet of the present invention may further comprise (1) Cu: 0.05 to 0.5%, Cr: 0.05 to 0.4%, Ni: 0.05 to 1%, Mo: 0.03. -0.3% and V: one or more selected from the group consisting of 0.005-0.1%, (2) Ca: 0.005% or less (excluding 0%) and / or rare earth elements: It may contain 0.003% or less (excluding 0%).

The high-tensile steel sheet of the present invention has a ferrite fraction of 30 area% or less, a tensile strength TS: 570 MPa or more, a yield point YP: 460 MPa or more, a brittle fracture surface transition temperature vTrs: −80 ° C. or less, and In the heat-affected zone imparted with a large heat input of 10 to 50 KJ / mm, the Charpy absorbed energy vE- _{20 at} −20 ° C. is preferably 100 J or more.

  In the present invention, in the steel sheet having a structure mainly composed of bainite, the chemical component composition is defined, and the crystal grain size of the large-angle grain boundary is predicted not only by average refinement but also predicted by an extreme value statistical method. By refining the maximum grain size that can be produced, it is possible to realize a high-strength steel sheet for high heat input welding that has excellent base metal strength and toughness and also has excellent HAZ toughness when subjected to high heat input welding. It is useful as a material for various structural materials including shipbuilding and bridges.

  In order to solve the above-mentioned problems, the inventor of the present invention pays attention to a steel sheet having a bainite structure in particular, and has excellent steel strength and toughness in the steel sheet and also has excellent HAZ toughness when subjected to high heat input welding. We examined from various angles to realize. As a result, the following knowledge was obtained. In the past technology, it was thought that by reducing the average value of the prior austenite grain size and crystal grains, the crack growth suppression effect was enhanced and the base material toughness (Charpy impact absorption characteristics) was improved. However, as shown in the examples described later, there are cases where such control cannot be arranged. Then, it was found that coarse crystal grains may exist only by achieving an average refinement of the structure, and these crystal grains serve as a starting point of fracture and reduce toughness.

The present inventor paid attention to such a phenomenon and proceeded with a study from the viewpoint of identifying the presence of coarse crystal grains. As a result, when the region surrounded by a large-angle grain boundary where the orientation difference between the two crystals is 15 ° or more is used as the crystal grain, the grain size of the crystal grain measured by the electron backscatter diffraction image method is expressed by the following (2) that - (7) predict a maximum particle diameter D _M calculated by extreme value statistics method based on expression, if such a 80μm or less, can avoid a situation in which there are coarse crystal grains, excellent base metal toughness I found out that

D _M = ay + b (2)
y = −ln {−ln [(T−1) / T]} (3)
T = (A _s + A) / A (4)
A = A ₀ / n (5)
y (i) = aD (i) + b (6)
y (i) = − ln {−ln [i / (n + 1)]} (7)
Where a, b: slope and intercept of equation (6), y: normalization variable, T: recursion period,
A _s : prediction target area (mm ² ), A: inspection reference area (mm ² ),
A ₀ : measurement area (mm ² ) of electron backscatter diffraction image method, n: number of observation fields,
D (i): crystal grain size measured by electron backscatter diffraction image method, showing the maximum size at A ₀ and i: observation field number.

  The above-mentioned “extremum statistical method” is known as a method for estimating the maximum value in an aggregate in which the distribution base is considered to decrease exponentially like a normal distribution or an exponential distribution. This is the method described in detail in “Metall fatigue, influence of minute defects and inclusions” (Yokendo 1993, pp. 233-236).

  The extreme value distribution is a distribution in which the maximum value and the minimum value of each set follow when a set of a certain number of data is extracted from data according to a certain basic distribution function. Even if the basic distribution is a normal distribution or an exponential distribution, the extreme value distribution is different from the basic distribution, and this distribution is called extreme value statistics. Among extreme value distributions, those that have a distribution function that can be considered to decrease exponentially in the base distribution function, such as normal distribution and exponential distribution, are called double exponential distributions. The target extreme value distribution follows a double exponent. Then, the maximum value can be estimated using extreme value statistics.

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 changed, and it was found that the toughness of the base metal becomes good at the grain boundaries having a certain crystal orientation difference. Then, the crystal orientation distribution, do it properly defined, including the predicted maximum particle size D _M, without the presence of coarse crystal grains, it is the better base metal toughness could be achieved.

However, in order to control the predicted maximum particle size D _M to 80μm or less, as a premise, when the orientation difference between adjacent crystals surrounded by 15 ° or more high-angle grain boundary region and the crystal grains, the The average equivalent circle diameter D _{A of the} crystal grains measured by the electron backscatter diffraction image method needs to be 10 μm or less (see FIG. 5 described later).

  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).

That is, at a position at a depth t / 4 from the surface (hereinafter sometimes referred to as “t / 4 portion”), the crystal grains are surrounded by a large-angle grain boundary with the orientation difference of 15 ° or more, and have the same area. with the average value of the diameter of a circle (circle equivalent diameter) (average equivalent-circle diameter D _a) and 10μm or less, by a 80μm below the predicted maximum particle size D _M, excellent base metal toughness A high-tensile steel plate was realized. In order to improve the base material characteristics in the steel sheet of the present invention, it was stipulated by each ship class including NK (Japan Maritime Association) that the crystal grain orientation relation was evaluated at t / 4 part. It is because it is a test object part.

  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”).

In addition, in the tensile properties, a high strength (yield point YP: 460 MPa or more, tensile strength TS: 570 MPa or more) can be realized by using a structure mainly composed of bainite. The phrase “mainly composed of bainite” may be a structure in which the ratio (fraction) of ferrite is 30 area% or less, but the ratio (fraction) of ferrite is controlled to 30 area% or less, and bainite is controlled. It is preferable that the ratio is 70 area% or more in order to achieve high strength. Further, combined with the control of the average value of the crystal grains (average equivalent circle diameter D _A ) and the predicted maximum particle diameter D _M , the base material toughness is improved (for example, −80 ° C. or less at the brittle fracture surface transition temperature vTrs). It will contribute to.

  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: 0.06 to 0.12%]
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 570 MPa (depending on the thickness of the steel sheet used), it is necessary to contain 0.06% or more. However, if the content exceeds 0.12%, the weldability deteriorates and the base metal toughness deteriorates. For these reasons, the C content is set to 0.06 to 0.12%. In addition, the minimum with preferable C content is 0.07%, and a preferable upper limit is 0.11%.

[Si: 0.05 to 0.5%]
Si is an element necessary for deoxidation and ensuring strength, and for that purpose, it is necessary to contain 0.05% or more. However, if the content exceeds 0.5%, weldability deteriorates. In addition, the minimum with preferable Si content is 0.10%, and a preferable upper limit is 0.4%.

[Mn: 1.0 to 1.8%]
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 1.0% or more. However, if it is contained excessively, weldability and crack sensitivity deteriorate, so it is necessary to make it 1.8% or less. In addition, the minimum with preferable Mn content is 1.5%, and a preferable upper limit is 1.7%.

[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: 0.025% 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. However, considering the degree of cleanliness of practical steel, it is suppressed to 0.025% or less. 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.

[Nb: 0.005 to 0.025%]
Since Nb has an effect of suppressing recrystallization of austenite, it can refine austenite grains and refine the structure after transformation. In order to exert such an effect, it is necessary to contain Nb in an amount of 0.005% or more (preferably 0.012% or more). However, since an excessive content impairs weldability, the Nb content is preferably 0.025% or less (preferably 0.020% or less).

[Ti: 0.005 to 0.03%]
Ti, like Nb, has the effect of suppressing recrystallization of austenite, and therefore exhibits the effect of refining austenite grains and refining the structure after transformation. 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).

[N: 0.002 to 0.009%]
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 0.002% or more (preferably 0.003% or more). In addition, the solid solution N causes the HAZ toughness to deteriorate. With the increase in the total nitrogen amount, the above-mentioned nitride increases, but the solute N also becomes excessive.

[B: 0.0005 to 0.003%]
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 0.003% or less.

In the high-strength steel sheet of the present invention, the chemical component composition needs to be controlled as described above, and the carbon equivalent Ceq defined by these components needs to be 0.40% or less.
Ceq (%) = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (1)
However, [C], [Mn], [Cr], [Mo], [V], [Cu] and [Ni] are the contents (mass of C, Mn, Cr, Mo, V, Cu and Ni, respectively). %).

The carbon equivalent Ceq defined by the above formula (1) is an index for the maximum hardness of the weld zone, and good toughness in HAZ when the heat input is a large heat input of 10 to 50 kJ / mm. In order to realize (for example, Charpy absorbed energy vE- _{20 at} −20 ° C. is 100 J or more), the carbon equivalent Ceq needs to be 0.40% or less. Too much HAZ toughness will deteriorate. In addition to the basic components C and Mn, the above-described formula (1) includes components (Cr, Mo, V, Cu and Ni) contained as necessary, as items in the formula. When the component is contained, the content may be taken into consideration and the value of the equation (1) may be calculated. When the component is not contained, the calculation may be performed without considering these components.

  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.

[Cu: 0.05-0.5%, Cr: 0.05-0.4%, Ni: 0.05-1%, Mo: 0.03-0.3% and V: 0.005-0 .1 or more selected from the group consisting of 1%]
These elements are effective in forming fine blocks by suppressing transformation and reducing Bs. Therefore, when these elements are contained, it is preferable to contain Cu, Cr, and Ni at 0.05% or more, Mo at 0.03% or more, and V at 0.005% or more. However, if the amount is excessive, weldability is impaired. Therefore, the upper limit in the case of containing these elements is determined as described above. Even if these elements are not actively contained, they may be inevitably mixed in due to the raw materials, etc., so if they are contained in an amount less than the lower limit, they are treated as inevitable impurities. It becomes.

[Ca: 0.005% or less (excluding 0%) and / or rare earth element (REM): 0.003% or less (excluding 0%)]
Ca and REM are effective elements for improving the toughness by fixing S, and in order to exert the effect, it is preferable to contain both 0.0005% or more. However, since the effect is saturated even if it contains excessively, it is preferable to make it 0.005% or less by Ca and 0.003% or less by REM. A more preferable lower limit when Ca is contained is 0.001%.

The high-strength steel sheet of the present invention appropriately controls the chemical component composition, and regulates the structure and crystal grains (average equivalent circle diameter D _A and predicted maximum grain diameter D _M ), thereby achieving the effects described above. However, in order to realize such a high-tensile steel plate, it may be manufactured according to the following method.

First, a steel slab that satisfies the requirements for the chemical composition is heated to a temperature range of 1000 to 1200 ° C., and then hot-rolled. The specific conditions for hot rolling are as follows: after rolling at a rolling reduction of 20% or more in the austenite recrystallization temperature range where the steel plate temperature is 950 to 900 ° C., the steel plate temperature is further lowered to 900 to 850 ° C. Rolling is completed at a recrystallization temperature range with a cumulative rolling reduction of 15% or more, and then accelerated cooling is performed at a cooling rate of 5 ° C./second or more above the Ar ₃ transformation point. In rolling in the austenite recrystallization temperature range where the steel plate temperature is 950 to 900 ° C., the TOM (Time Out Mill) of each pass is preferably 80 seconds or less.

  In order to exhibit the effect of the chemical component of the present invention, the heating temperature needs to be 1000 ° C. or higher so that the solid solution amount of Nb is 0.005% or higher. However, if the heating temperature becomes too high, the initial austenite grain size becomes coarse, and the transformation structure cannot be made sufficiently fine.

The reason why the cumulative rolling reduction in the low-temperature recrystallization temperature range where the steel sheet temperature is 900 to 850 ° C. is 15% or more is that it is effective to refine the average size of the large-angle grain boundaries, which is effective for suppressing the progress of cracks. (See Fig. 8 below). However, in order to sufficiently refine the predicted maximum particle diameter _DM that is the starting point of fracture, rolling in this temperature range may not be sufficient (see FIG. 9 described later).

To sufficiently fine the predicted maximum particle size D _M, the cumulative reduction rate at a low temperature recrystallization temperature region of nine hundred to eight hundred fifty ° C.: before performing 15% or more rolling, the steel sheet temperature is nine hundred and fifty to nine hundred ° C. It is effective to perform rolling at a cumulative reduction ratio of 20% or more in the austenite recrystallization temperature range. However, when the TOM in this temperature range exceeds 80 seconds, the refined austenite grains (γ grains) return to the original size by recrystallization, and as shown in FIG. Even when rolling is performed as described above, it may become coarse.

The cumulative rolling reduction is a value calculated from the following equation (8).
Cumulative rolling reduction = (t ₀ −t ₁ ) / t ₀ × 100 (8)
[In the formula (8), t ₀ represents the rolling start thickness (mm) of the steel slab when the temperature at the t / 4 position of the steel slab is within the rolling temperature range, and t ₁ represents the t / 4 position of the steel slab. The rolling end thickness (mm) of the steel slab when the temperature is within the rolling temperature range, respectively. ]

Further, in the present invention, the value obtained by the following equation (9) is basically adopted as the Ar ₃ transformation point shown in the above production conditions.
Ar ₃ transformation point = 910-310 [C] -80 [Mn]-[Cu] -15 [Cr] -55 [Ni] -80 [Mo] +0.35 (t-8) (9)
About the meaning of each item, it is the same as said (1) Formula.

By rolling in the non-recrystallization temperature range and introducing strain into the austenite grains, the subsequent transformation structure can be refined, but the effect is not as great as in the reduction in the recrystallization temperature range. Therefore, it is preferable to finish rolling when the steel plate (at least t / 4 part) is not less than the Ar ₃ transformation point without expecting a refinement effect in the non-recrystallization temperature region. Thereby, the ferrite fraction in the structure can be reduced to 30% by area or less.

The reason why the cooling rate after rolling (cooling rate at or above the Ar ₃ transformation point) is set to 5 ° C./second or more (for example, water cooling) is to sufficiently refine the microstructure after transformation to achieve high strength. The cooling stop temperature at this time needs to be 450 ° C. or lower in order to obtain a structure mainly composed of bainite.

  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 having the chemical composition shown in Table 1 (steel types A to H) were melted in a converter and rolled under the conditions shown in Table 2 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.

[Temperature measurement method during rolling]
1. In the process computer, the heating temperature at a predetermined position (t / 4 part) 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, ferrite fraction (alpha fraction), prior austenite grain size (old γ grain size), large angle grain boundary diameter (average equivalent circle diameter D _A and predicted maximum grain size D _M ), tensile properties ( Yield point YP, tensile strength TS), base material toughness (base material impact characteristics), and HAZ toughness (high heat input characteristics) were measured by the following methods (all t / 4 parts).

[Measurement of ferrite fraction (α fraction)]
A sample was cut out from the 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 with wet emery polishing paper from # 150 to # 1000 And then mirror finished using diamond slurry as an abrasive. After this mirror-polished piece was etched with a 2% nitric acid-ethanol solution (Nital solution), a 150 μm × 200 μm field of view was observed at an observation magnification of 400 times, and the ferrite 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 prior austenite grain size (old γ grain size)]
1. A sample including the front and back surfaces of the plate thickness cut in a direction parallel to the rolling direction of the steel plate is prepared.
2. A mirror finish is applied using a wet emery abrasive paper of # 150 to # 1000 or a polishing method (abrasive such as abrasive paper or diamond slurry) having the same function.
3. The mirror-polished piece was etched with an ultra-low carbon corrosive solution, the old γ grain boundary was taken out, and a 300 μm × 400 μm field of view was observed at an observation magnification of 200 times for image analysis.
4). Image analysis measures the maximum width (usually along the thickness direction) and the maximum length (usually along the rolling direction) at each grain boundary, and calculates the equivalent circle diameter from these, Was the old γ particle size.

[Measurement of average size of large-angle grain boundary diameter (equivalent circle diameter D _A )]
1. A sample including the front and back surfaces of the plate thickness cut in a direction parallel to the rolling direction of the steel plate is prepared.
2. A mirror finish is applied using a wet emery abrasive paper of # 150 to # 1000 or a polishing method (abrasive such as abrasive paper or diamond slurry) having the same function.
3. At the position of t / 4, 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 EF-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 average value of the large-angle grain boundary diameters thus obtained was calculated to obtain the average equivalent circle diameter D _A (hereinafter sometimes referred to as “average large-angle grain boundary size”) in the present invention.
4). As an analysis method of the text data, those having a crystal grain size of 2.5 μm or less were judged as measurement noise and excluded from the average value calculation target.

[Measurement of maximum particle size of large-angle grain boundary diameter (predicted maximum particle size D _M )]
1. The text data obtained at the time of crystal grain measurement by the EBSP is measured for 10 visual fields (n = 10) at different portions of t / 4.
2. The inspection standard area: A is the observation visual field by EBSP: A ₀ , and the maximum crystal grain size (maximum size): D (i) in A _{0 is} extracted from the text data for 10 visual fields.
3. The ten maximum crystal grain sizes measured: D (i) are arranged in order from the smallest, and D (1) to D (10). In addition, the cumulative distribution function: F (i) and the normalization variable: y (i) are calculated by the following formulas (10) and (11), for example, as shown in Table 3 below (example of extreme value statistical calculation). To summarize.
y (i) =-In {-In [i / (n + 1)]} (10)
F (i) = i / (n + 1) × 100 (11)

4). D (i) is plotted on the horizontal axis, F (i) and y (i) are plotted on the vertical axis, and a line is drawn by the method of least squares. In FIG. Examples of 1, 3 (conditions No. 1 and 3, respectively) are shown. It is confirmed that the extreme value distribution obtained at this time is on a straight line in the region of 10 to 85% on the F scale. If it rides, it can be considered that it follows a double exponential distribution, that is, it is possible to estimate the maximum value using extreme value statistics. It should be noted that the number of measurement fields: n is considered to be more reliable when the number is larger, but it can be determined that the measurement efficiency only decreases if the number is increased too much. A sufficient maximum particle size was considered predictable.

5. The above straight line shows the above equation (6), and the inclination and intercept are a and b, respectively. Further, the standardization variable y is obtained by the above equations (3) to (5), and finally, based on the equation (2), the predicted maximum particle size D _M (hereinafter referred to as “predicted maximum large angle grain boundary size”). May be calculated).

[Tensile properties of base material]
An NK U14A test piece was collected from the t / 4 part of each steel plate, and the yield point YP and the tensile strength TS were measured by performing a tensile test according to JIS Z2241. The yield point criterion was 460 MPa or more, and the tensile strength criterion was 570 MPa or more.

[Evaluation of impact characteristics of base material]
For the impact characteristics of the base material, a V-notch Charpy test was performed, and vTrs (fracture surface transition temperature) was obtained from a transition curve. NK U14A test specimens were collected from t / 4 parts and tested according to JIS Z2242. At this time, for each temperature (measurement of at least 4 temperatures), the test was conducted with n = 3, and a brittle fracture surface transition curve was drawn so as to pass through the point having the highest fracture surface ratio among the three points, and the fracture surface ratio was 50 % Temperature was calculated as the brittle fracture surface transition temperature vTrs (a line was drawn so that vTrs was on the highest temperature side). For example, taking FIG. 2 (fracture surface transition temperature calculation method explanatory diagram) as an example, when a brittle fracture surface transition curve is drawn so as to pass through a point having the highest fracture surface ratio, it varies at −60 ° C. = −55 ° C. In the shipbuilding E grade in the NK ship class, the impact characteristics of the base material are required to be a test temperature of −40 ° C. Therefore, in order to stably satisfy the low temperature impact characteristics including variations, the vTrs is determined to be −80 ° C. or less.

[Evaluation of large heat input characteristics]
The butt groove is subjected to one-pass welding by electrogas welding (EGW) (heat input 30 to 50 KJ), and NK U4 so that the center of the test piece is 7 mm in the plate thickness direction from the root side (back side) A test piece was collected, a notch was made in the fusion line portion, and the test was performed in accordance with JIS Z 2242. The reason for adopting the route side is that the heat history is the severest in the thickness direction. Test temperature: Three impact tests were conducted at −20 ° C., and the average absorbed energy vE ₋₂₀ Ave was determined to be 100 J or more.

  These results are shown in the following Tables 4 and 5 together with the production conditions and the welding method, and can be considered as follows from these results. First, experiment no. 3 to 5, 11, 13, 16, 19 and 23 satisfy the requirements defined in the present invention, and it is understood that the base material characteristics and the HAZ toughness are good. Those lacking any of the requirements defined in the present invention (Experiment Nos. 1, 2, 6 to 10, 12, 14, 15, 17, 18, 20 to 22, 24 to 27) are deteriorated. You can see that

  Based on these results, FIG. 3 shows the relationship between the γ particle size at t / 4 part and vTrs, and FIG. 4 shows the relationship between the average large-angle grain boundary size at t / 4 part and vTrs. It can be seen that the toughness of the base metal is not improved only by controlling the average large-angle grain boundary size.

  FIG. 5 shows the relationship between the average large-angle grain boundary size and the predicted maximum large-angle grain boundary size at t / 4 part. To at least make the predicted maximum large-angle grain boundary size 80 μm or less, the average large-angle grain boundary size is 10 μm or less. It is understood that it is necessary to.

  FIG. 6 shows the relationship between the predicted maximum large-angle grain boundary size and vTrs at the t / 4 part. It can be seen that the toughness of the base material is improved by reducing the predicted maximum large-angle grain boundary size. .

  FIG. 7 shows the relationship between the α fraction at t / 4 part and the mechanical properties (YP, TS), and it can be seen that the strength of the base material is improved by reducing the α fraction.

  FIG. 8 shows the relationship between the cumulative rolling reduction at 900 to 850 ° C. and the average large angle grain boundary size at t / 4 part. By setting the cumulative rolling reduction at 900 to 850 ° C. to 15% or more, the average large angle It can be seen that refinement of the grain boundary size has been achieved.

  FIG. 9 shows the relationship between the cumulative rolling reduction at 900 to 850 ° C. and the predicted maximum large-angle grain boundary size at the t / 4 part, but the prediction is made only by setting the cumulative rolling reduction at 900 to 850 ° C. to 15% or more. It turns out that refinement | miniaturization of the largest large angle grain boundary size may not be achieved.

  FIG. 10 shows the relationship between the cumulative rolling reduction at 950 to 900 ° C. and the predicted maximum large-angle grain boundary size at t / 4 part (however, the cumulative rolling reduction at 900 to 850 ° C. is 15% or more) It can be seen that even when the cumulative rolling reduction at 950 to 900 ° C. is 20% or more, if the TOM is longer than 80 seconds, the predicted maximum large-angle grain size may not be refined.

Experiment No. It is a graph which shows the example of extreme value statistics calculation in the case of 1 and 3. It is a figure explaining the method of calculating fracture surface transition temperature vTrs. It is a graph which shows the relationship between (gamma) particle size and vTrs in t / 4 part. It is a graph which shows the relationship between the average large angle grain boundary size in v / 4 part, and vTrs. It is a graph which shows the relationship between the average large angle grain boundary size in t / 4 part, and the prediction maximum large angle grain boundary size. It is a graph which shows the relationship between the prediction maximum large angle grain boundary size in v / 4 part, and vTrs. It is a graph which shows the relationship between (alpha) fraction in t / 4 part, and mechanical characteristics (YP, TS). It is a graph which shows the relationship between the cumulative reduction rate of 900-850 degreeC, and the average large angle grain boundary size in t / 4 part. It is a graph which shows the relationship between the cumulative rolling reduction of 900-850 degreeC, and the prediction maximum large angle grain boundary size in t / 4 part. It is a graph which shows the relationship between the cumulative reduction ratio of 950-900 degreeC, and the prediction largest large angle grain boundary size in t / 4 part.

Claims (4)

C: 0.06 to 0.12% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0. 06%, P: 0.025% or less (not including 0%), S: 0.01% or less (not including 0%), Nb: 0.005 to 0.025%, Ti: 0.005 0.03%, N: 0.002 to 0.009% and B: 0.0005 to 0.003%, respectively, and the carbon equivalent Ceq defined by the following formula (1) is 0.40% or less The balance consists of iron and inevitable impurities,
The bainite phase is 70 area% or more, and the balance is a ferrite structure,
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 (t represents the plate thickness, the same applies hereinafter) from the surface, the crystal The average equivalent circular diameter D _{A of the} grains measured by electron backscatter diffraction imaging is 10 μm or less,
The predicted maximum particle size D _M calculated by the extreme value statistical method based on the following formulas (2) to (7) for the crystal grain size measured by the electron backscatter diffraction image method is 80 μm or less. High strength steel plate for high heat input welding with excellent base metal low temperature toughness.
Ceq (%) = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15 (1)
However, [C], [Mn], [Cr], [Mo], [V], [Cu] and [Ni] are the contents (mass of C, Mn, Cr, Mo, V, Cu and Ni, respectively). %).
D _M = ay + b (2)
y = −ln {−ln [(T−1) / T]} (3)
T = (A _s + A) / A (4)
A = A ₀ (5)
y (i) = aD (i) + b (6)
y (i) = − ln {−ln [i / (n + 1)]} (7)
Where a, b: slope and intercept of equation (6), y: normalization variable, T: recursion period,
A _s : prediction target area (mm ² ), A: inspection reference area (mm ² ),
A ₀ : measurement area (mm ² ) of electron backscatter diffraction image method, n: number of observation fields,
D (i): crystal grain size measured by electron backscatter diffraction image method, showing the maximum size at A ₀ and i: observation field number.   Furthermore, Cu: 0.05-0.5%, Cr: 0.05-0.4%, Ni: 0.05-1%, Mo: 0.03-0.3% and V: 0.005- The high-tensile steel plate for high heat input welding according to claim 1, which contains at least one selected from the group consisting of 0.1%.   The large input according to claim 1 or 2, further comprising Ca: 0.005% or less (not including 0%) and / or rare earth elements: 0.003% or less (not including 0%). High strength steel plate for heat welding. In the heat-affected zone where the tensile strength TS is 570 MPa or more, the yield point YP is 460 MPa or more, the brittle fracture surface transition temperature vTrs is −80 ° C. or less, and a large heat input with a heat input of 10 to 50 kJ / mm is applied. The high tensile strength steel plate for high heat input welding according to any one of claims 1 to 3, wherein Charpy absorbed energy vE- _{20 at} 20 ° C is 100 J or more.

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