JP4044470B2 - High toughness steel sheet excellent in low temperature base metal toughness and low temperature HAZ toughness, and method for producing the same - Google Patents

Description

【００５５】
【表２】

【００５６】
【表３】

【００５７】
【表４】

【００５８】
【表５】

【００５９】
表４および５から、次のように考察できる。評価用鋼板Ｎｏ．３，４，６、１２〜２２は、いずれも化学成分が良好な表１の鋼種番号１〜１２を用いると共に、表４に示すように組織が良好であり、低温母材靭性、低温ＨＡＺ靭性を始めとする各種母材特性および溶接性が良好であった。これに対し、評価用鋼板Ｎｏ．１，２，５，７〜１１，２３〜３９は、表２に示すように製造条件が不適であるために表４に示すように組織が不適であるか（評価用鋼板Ｎｏ．１，２，５，７〜１１）、化学成分が不適な表１の鋼種番号１３〜２９を用いているために表５に示すように組織が不適（評価用鋼板Ｎｏ．２３〜３９）であり、以下の不具合を有している。
【００６０】
Ｎｏ．１の評価用鋼板は熱間圧延前に均質化圧延処理を施していない例、Ｎｏ．２の評価用鋼板は均質化圧延処理時の加熱温度が低い例である。Ｎｏ．５の評価用鋼板は焼戻し温度が高い例である。Ｎｏ．７の評価用鋼板は１０００から９００℃までの累積圧下率が低い例、Ｎｏ．８の評価用鋼板は９００から７００℃までの累積圧下率が低い例である。Ｎｏ．９の評価用鋼板は熱間圧延後の冷却速度が遅い例、Ｎｏ．１０の評価用鋼板は熱間圧延後の冷却停止温度が高い例である。Ｎｏ．１１の評価用鋼板は熱間圧延時の加熱温度が高い例である。これらの評価用鋼板では、平均結晶粒径が大きく、低温母材靭性が劣っており、さらにＮｏ．９および１０の評価用鋼板では、母材強度も劣っている。
【００６１】
Ｎｏ．２３の評価用鋼板はＣ量が少ない例であり、ベイナイト分率が低く、平均結晶粒径が大きい。その結果、母材強度および低温母材靭性が劣っている。Ｎｏ．２４の評価用鋼板はＣ量が多い例であり、低温ＨＡＺ靭性が劣っている。
【００６２】
Ｎｏ．２５の評価用鋼板はＳｉ量が多い例であり、低温母材靭性および低温ＨＡＺ靭性が劣っている。
【００６３】
Ｎｏ．２６の評価用鋼板はＭｎ量が少ない例であり、ベイナイト分率が低く、平均結晶粒径が大きい。その結果、母材強度および低温母材靭性が劣っている。
【００６４】
Ｎｏ．２７の評価用鋼板はＭｎ量が多い例、Ｎｏ．２８および２９の評価用鋼板はＫＰが高い例、Ｎｏ．３０の評価用鋼板はＣｒ量が好ましい範囲を超えており且つＫＰが大きい例、Ｎｏ．３１の評価用鋼板はＭｏ量が多い例である。これらの評価用鋼板では、低温ＨＡＺ靭性が劣っている。
【００６５】
Ｎｏ．３２の評価用鋼板はＮｂ量が多い例、Ｎｏ．３３の評価用鋼板はＶ量が好ましい範囲を超える例である。これらの評価用鋼板では、平均結晶粒径が大きく、低温母材靭性および低温ＨＡＺ靭性が劣っている。
【００６６】
Ｎｏ．３４の評価用鋼板はＮｉおよびＣｕが多い例であり、低温ＨＡＺ靭性が劣っている。
【００６７】
Ｎｏ．３５の評価用鋼板はＢ量が少なく、ベイナイト分率が低い例であり、母材強度が劣っている。Ｎｏ．３６の評価用鋼板はＢ量が多く、平均結晶粒径が大きい例であり、低温母材靭性および低温ＨＡＺ靭性が劣っている。
【００６８】
Ｎｏ．３７の評価用鋼板はＮ量が多い例、Ｎｏ．３９の評価用鋼板はＴｉ量が多い例であり、これらの評価用鋼板では平均結晶粒径が大きく、低温母材靭性および低温ＨＡＺ靭性が劣っている。Ｎｏ．３８の評価用鋼板はＴｉ量が少ない例であり、低温ＨＡＺ靭性が劣っている。
【００６９】
【発明の効果】
本発明は以上のように構成されており、特定の化学組成（特にＫＰ）に加えて、鋼組織のベイナイト分率および平均結晶粒径を特定の範囲とすることで、低温母材靭性および低温ＨＡＺ靭性（大入熱ＨＡＺ靭性）に優れた４９０ＭＰａ級以上の鋼板を提供することができた。本発明の鋼板は、例えば、船舶や海洋構造物など、特に低温環境下に曝される構造物（溶接構造物）用途に好適である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a steel plate having a tensile strength of 490 MPa or higher (hereinafter referred to as a steel plate of 490 MPa class or higher) excellent in base metal toughness at low temperature and excellent in weldability (high heat input HAZ toughness, weld crack resistance). It is about. The steel sheet of the present invention is mainly applied to welded structures such as ships and offshore structures.
[0002]
[Prior art]
Steel plates (thick steel plates) applied to ships, offshore structures, and the like are required to have excellent base material toughness at low temperatures in addition to high base material strength as well as their application environment. In steel sheets applied in such fields, weldability (high heat input HAZ toughness and weld crack resistance) is also required as an important characteristic.
[0003]
In such a steel sheet, various techniques for increasing the HAZ (welding heat affected zone) toughness (particularly high heat input HAZ toughness) while taking into account the securing of the base material toughness have been proposed. A technique is disclosed in which austenite grains are refined by utilizing pinning of oxide inclusions to ensure HAZ toughness.
[0004]
Further, Patent Document 2 suppresses the formation of intergranular ferrite by solute B, secures the base metal toughness with the steel structure as the main acicular ferrite, and utilizes the ferrite nucleation ability of CaS and MnS, A technique for improving the HAZ toughness is disclosed.
[0005]
Although these techniques ensure the base material toughness and the HAZ toughness at the −20 ° C. level and the −40 ° C. level, recently, in the base material toughness and the HAZ toughness of the steel sheet applied in the above field, for example, There is also a demand at a −60 ° C. level, and the steel sheets disclosed in Patent Document 1 and Patent Document 2 cannot sufficiently meet such a demand.
[0006]
[Patent Document 1]
JP 2001-89825 A [Patent Document 2]
Japanese Patent Laid-Open No. 2002-235114
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and the object thereof is a steel plate having a strength of 490 MPa or more excellent in toughness (base metal toughness and large heat input HAZ toughness) at a low temperature (for example, −60 ° C.). Is to provide.
[0008]
[Means for Solving the Problems]
The high toughness steel sheet of the present invention that can achieve the above object is C: 0.010 to 0.070% (meaning of mass%, the same shall apply hereinafter), Si: 0.8% or less, Mn: 1.0 to 1 0.9%, Nb: 0.032% or less (including 0%), Ti: 0.005-0.10%, B: 0.0006-0.0050%, N: 0.0020-0.010% , Satisfying KP <2.4, having a gist where 50% by volume or more of the steel structure is bainite and having an average crystal grain size of 8 μm or less, at a low temperature. Excellent toughness (base metal toughness and high heat input HAZ toughness).
[0009]
Here, the KP is expressed by the following formula (1).
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] (1)
<< In formula, [] means content (mass%) of each element. >>
[0010]
In the high toughness steel sheet of the present invention, Ni: 2.0% or less, or Cu: 2.0% or less; further, Cr: 1.0% or less, or Mo: 0.30% or less, or V: 0. Al: 0.20% or less; P: 0.020% or less; S: 0.010% or less; Ca: 0.0050% or less; Mg: 0.005% or less Or REM (rare earth element): 0.02% or less, or Zr: 0.050% or less, or P: 0.020% or less, S: 0.010% or less is also effective The properties of the high toughness steel sheet are further improved according to the types of these components. Such a high toughness steel sheet of the present invention has good characteristics even when the wall thickness is 50 mm or more.
[0011]
The chemical composition of the high toughness steel sheet according to the present invention typically consists of the balance of the above elements and the remaining Fe and unavoidable impurities, but other chemical components also do not impair the effects of the present invention. It may be contained.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have made extensive studies to develop a steel plate (thick steel plate) of 490 MPa class or more that is particularly excellent in low-temperature (for example, −60 ° C.) base metal toughness and high heat input HAZ toughness. The invention has been completed.
[0013]
In securing high heat input HAZ toughness, C should be limited to low C, and Ti, which can contribute to refinement of the HAZ structure, and B, which can form nucleation sites of ferrite, should be added in an appropriate amount. Furthermore, the base material strength was set to 490 MPa or more by adding a non-carbide-generating element (Mn, preferably Cu, Ni, Cr, etc.) that does not relatively deteriorate the HAZ toughness, or by adding a small amount of Nb. However, it has been found that when the KP represented by the above formula (1) exceeds a specific value, the low-temperature HAZ toughness deteriorates. Therefore, the low temperature HAZ toughness was also secured by further controlling this KP. As for the base material toughness at low temperature, the refinement of crystal grains was achieved by controlling the solid solution B amount and optimizing the rolling conditions, thereby ensuring excellent low temperature base material toughness.
[0014]
Since solute B has high hardenability, it is effective in securing strength, but at the same time, it has been found that low C has an action of coarsening crystal grains. On the other hand, as described above, since B is an element that greatly contributes to the toughening of HAZ, it is not preferable to limit the addition amount of B. Therefore, by optimizing the rolling conditions, the control of the processing austenite is performed together with the control of the amount of dissolved B, the hardenability (that is, the base material strength), the refinement of the crystal grains (that is, the base material toughness, particularly at low temperatures). It has succeeded in securing all of the base material toughness) and the low temperature HAZ toughness at a high level. Hereinafter, the basic components and structure of the steel sheet of the present invention will be described.
[0015]
C: 0.010-0.070%
C is an important element for achieving both the weld crack resistance of the HAZ part during welding and the strength of the base material and improving the high heat input HAZ toughness. When C exceeds 0.070%, martensite is generated instead of low-temperature transformation bainite on the high cooling rate side, and the weld crack resistance is not improved. Further, on the low cooling rate side (high heat input HAZ), a large amount of martensite and austenite composite structure (hereinafter referred to as “MA”) is generated, and the high heat input HAZ toughness is not improved. Preferably it is 0.060% or less, More preferably, it is 0.055% or less. In addition, if it is less than 0.010%, the necessary minimum base material strength cannot be obtained. Preferably it is 0.020% or more, More preferably, it is 0.030% or more.
[0016]
Si: 0.8% or less Si is an element useful as a deoxidizer, but if added over 0.8%, the weldability and the base metal toughness deteriorate, so the upper limit was defined as 0.8%. Preferably it is 0.6% or less, More preferably, it is 0.3% or less.
[0017]
Mn: 1.0 to 1.9%
Mn has an effect of improving quenching and also has an effect of improving the toughness of the base metal by refining crystal grains. However, if it exceeds 1.9%, the weld crack resistance of the HAZ part is lowered. Preferably it is 1.8% or less, More preferably, it is 1.60% or less. On the other hand, if the Mn is less than 1.0%, sufficient base material strength cannot be obtained. Preferably it is 1.20% or more, more preferably 1.3% or more.
[0018]
Nb: 0.032% or less (including 0%)
Nb has the effect of significantly improving the hardenability due to the combined effect with B. However, if it exceeds 0.032%, the crystal grains become coarse, and the base metal toughness and the HAZ toughness are lowered. Preferably it is 0.025% or less, More preferably, it is 0.018% or less. On the other hand, in order to effectively exhibit the effect of adding Nb, the content is preferably 0.005% or more, and more preferably 0.008% or more.
[0019]
Ti: 0.005-0.10%
Ti forms nitrides with N to refine γ grains in the HAZ part, and becomes a BN generation site, promotes the formation of intragranular ferrite, and has the effect of greatly improving the HAZ toughness. If it is less than 005%, such an effect cannot be secured sufficiently. Preferably it is 0.007% or more, More preferably, it is 0.009% or more. On the other hand, when Ti exceeds 0.10%, both HAZ toughness and base metal toughness deteriorate. Preferably it is 0.025% or less, More preferably, it is 0.020% or less.
[0020]
B: 0.0006 to 0.0050%
B has the effect of improving the hardenability by being dissolved, but when the amount of the solid solution is too large, the toughness is impaired. Moreover, in HAZ, it becomes BN and works as a nucleation site of ferrite and has an effect of improving HAZ toughness. If the amount of B is less than 0.0006%, the effect of adding B cannot be secured sufficiently. Preferably it is 0.0010% or more, More preferably, it is 0.0012% or more. On the other hand, when the amount of B exceeds 0.0050%, the hardenability is lowered and the base material toughness and the HAZ toughness are deteriorated. Preferably it is 0.0030% or less, More preferably, it is 0.0025% or less.
[0021]
N: 0.0020 to 0.010%
N forms Ti and nitride to refine the γ grain size of the HAZ part, and N forms B and nitride to promote the formation of ferrite in the HAZ part and has the effect of improving the HAZ toughness. If the N content exceeds 0.010%, both the base metal toughness and the HAZ toughness deteriorate. Preferably it is 0.0060% or less. On the other hand, if the N content is less than 0.0020%, the effect of improving the HAZ toughness due to the formation of nitride with Ti becomes insufficient. Preferably it is 0.0030% or more.
[0022]
KP <2.4
KP is an index indicating the ease with which MA can be produced with low carbon bainite. When KP is 2.4 or more, the amount of MA produced is excessive and HAZ toughness (especially low-temperature HAZ toughness) deteriorates. From the viewpoint of improving the HAZ toughness, the smaller the value of KP, the more desirable it is, for example, 2.0 or less, more preferably 1.7 or less.
[0023]
Bainite fraction: 50% by volume or more Bainite is a structure that is effective in increasing the strength of the base metal. The higher the bainite fraction of the steel structure, the higher the base metal strength. That is, when the bainite fraction is less than 50% by volume, sufficient base material strength cannot be ensured. Preferably it is 75 volume% or more, More preferably, it is 90 volume% or more. In the present invention, the bainite includes a tempered bainite structure.
[0024]
Average crystal grain size: 8 μm or less As described above, the smaller the average crystal grain size in the steel sheet, the better the base metal toughness. That is, if the average crystal grain size exceeds 8 μm, the base material toughness becomes insufficient. Preferably it is 6.5 micrometers or less, More preferably, it is 5 micrometers or less.
[0025]
The “average crystal grain size” in the present invention is measured by FE-SEM-EBSP (electron backscatter diffraction image method using a field emission scanning electron microscope) in a cross section parallel to the rolling direction of the steel sheet. Value. Specifically, an EBSP analyzer (such as an EBSP analyzer manufactured by TexSEM Laboratories) is used in combination with an FE-SEM, and the crystal grain size is determined with a boundary having an inclination (crystal orientation difference) of 10 degrees or more as a grain boundary. decide. As measurement conditions, the measurement area is 100 μm, the measurement steps are 0.4 μm intervals, and measurement points with a confidence index (Confidence Index) indicating the reliability of the measurement direction are 0.1 or less are deleted from the analysis target. The average value of the crystal grain sizes thus obtained is calculated and used as the average crystal grain size of the present invention. A crystal grain size of 1.2 μm or less is determined as measurement noise and excluded from the target of calculating the average value of crystal grain sizes.
[0026]
Furthermore, in the present invention, it is recommended to actively add the following elements or suppress the content thereof in order to improve various properties.
[0027]
Ni: 2.0% or less Ni is an element useful for improving the toughness of the base metal. However, if the content exceeds 2.0%, the HAZ toughness tends to deteriorate instead, so the upper limit is 2.0%. It is preferable to do. More preferably, it is 1.5% or less, More preferably, it is 0.9% or less.
[0028]
Cu: 2.0% or less Cu has an effect of improving the hardenability as well as improving the strength of the base metal by solid solution strengthening and precipitation strengthening. However, if it exceeds 2.0%, the high heat input HAZ toughness tends to decrease, so the upper limit is preferably made 2.0%. More preferably, it is 1.2% or less, More preferably, it is 0.9% or less.
[0029]
Cr: 1.0% or less Cr has the effect of improving the base metal strength by improving the hardenability, but if it exceeds 1.0%, the amount of MA generated tends to increase and the HAZ toughness tends to deteriorate, so its upper limit Is preferably 1.0%. More preferably, it is 0.5% or less, More preferably, it is 0.3% or less.
[0030]
Mo: 0.30% or less Mo has the effect of improving the hardenability and improving the base metal strength, but also has the effect of greatly degrading the HAZ toughness, so the upper limit is made 0.30% or less. It is preferable. More preferably, it is 0.15% or less, More preferably, it is 0.10% or less.
[0031]
V: 0.10% or less V has the effect of increasing hardenability and temper softening resistance by addition of a small amount, but if it exceeds 0.10%, the base metal toughness and HAZ toughness tend to decrease, so the upper limit Is preferably 0.10%. More preferably, it is 0.06% or less, More preferably, it is 0.02% or less.
[0032]
Al: 0.20% or less Al is an effective deoxidizing element, but if it is contained in excess of 0.2%, the base material toughness and the HAZ toughness tend to decrease, so the upper limit is 0.20%. It is preferable to do. More preferably, it is 0.1% or less, More preferably, it is 0.05% or less.
[0033]
P: 0.020% or less, S: 0.010% or less P and S are impurities, and are preferably suppressed to 0.020% or less and 0.010% or less, respectively. More preferably, P is 0.010% or less, and S is 0.005% or less.
[0034]
Ca: 0.0050% or less Ca has the effect of spheroidizing MnS and reducing the anisotropy of inclusions. In order to fully exhibit this effect, it is preferable to add 0.0005% or more of Ca. More preferably, it is 0.001% or more. On the other hand, if the Ca content exceeds 0.0050%, the base material toughness tends to decrease, so the upper limit is preferably made 0.0050%. More preferably, it is 0.003% or less.
[0035]
Mg: 0.005% or less, REM: 0.02% or less, Zr: 0.05% or less These elements have an action of improving HAZ toughness, but if they are contained excessively, the HAZ toughness tends to deteriorate. Therefore, Mg: 0.005% or less, REM: 0.02% or less, and Zr: 0.05% or less are preferable. More preferably, Mg: 0.003% or less, REM: 0.01% or less, Zr: 0.03% or less. The REM that may be contained in the steel sheet of the present invention includes any of scandium (Sc), yttrium (Y), and lanthanoid series rare earth elements (atomic numbers 57 to 71) belonging to Group 3 of the periodic table. Can be used.
[0036]
As described above, the steel sheet of the present invention having the above chemical composition and structure is a so-called 490 MPa grade or more steel sheet, and the specific base material strength is 490 MPa or more in terms of tensile strength measured in Examples described later. Preferably there is. Also, if the tensile strength is too large, the elongation tends to decrease, so it is recommended that the upper limit of the tensile strength be 690 MPa.
[0037]
In manufacturing the steel sheet of the present invention, a steel having the above chemical composition is usually used except for considering the manufacturing conditions for satisfying the above-described structure (bainite fraction and average crystal grain size). What is necessary is just to employ | adopt suitably the manufacturing process and conditions (temperature, time, etc.) of a steel plate (thick steel plate). Below, the manufacturing method suitable in order to ensure the above-mentioned bainite fraction and average crystal grain diameter is demonstrated.
[0038]
First, a homogenization rolling process is provided before a hot rolling process (main rolling process). This homogenization rolling process is provided as a pretreatment process for controlling the amount of dissolved B in the hot rolling process. In this process, N fixed to Ti or Nb is decomposed as much as possible so that BN can be easily formed in the hot rolling process, so that the amount of dissolved B can be controlled. That is, in the hot rolling step, as described later, it is preferable to suppress the upper limit of the heating temperature to some extent from the viewpoint of securing the toughness of the steel sheet, but at such a temperature, N fixed to Ti or Nb is decomposed. Therefore, in the homogenization rolling process provided prior to the hot rolling process, the steel sheet is heated to a temperature at which N can be decomposed, and N is added to the heating temperature range of the next hot rolling process. The form is easy to form. From such a viewpoint, it is preferable that the heating temperature in the homogenization rolling step is 1150 ° C. or higher. The upper limit of the heating temperature is recommended to be 1350 ° C.
[0039]
Next, although it is a hot rolling process, it is preferable to make heating temperature into 1050-1150 degreeC. If the heating temperature is too low, B, Nb and Ti are not dissolved, and the hardenability may be lowered. On the other hand, if the heating temperature is too high, the γ particle size at the time of heating becomes coarse and the toughness may be deteriorated.
[0040]
The rolling reduction during hot rolling is a cumulative rolling reduction from 1000 ° C to 900 ° C (ratio to the thickness before rolling in the temperature range, the same applies hereinafter) of 40% or more, and further from 900 ° C to 700 ° C. The cumulative rolling reduction is desirably 60% or more. By setting the cumulative rolling reduction from 1000 ° C. to 900 ° C. to 40% or more, the heated γ grains can be recrystallized, and the γ grains can be refined and abnormal grain growth can be suppressed. In addition, by setting the cumulative reduction ratio from 900 ° C. to 700 ° C. to 60% or more, the flatness of γ grains that do not recrystallize increases, the γ grain boundary curves, and the nucleation sites of bainite increase. To do. As a result, the crystal grains become finer and good base material toughness can be secured.
[0041]
The rolling end temperature of the hot rolling is desirably 850 to 700 ° C. Above 850 ° C., the nucleation sites of bainite introduced by rolling (curvature of γ grain boundaries, etc.) tend to decrease, leading to deterioration of toughness. In addition, since ferrite is generated at 700 ° C. or lower, the strength of the base material may not be ensured.
[0042]
In the cooling after hot rolling, the cooling rate is preferably 3 ° C./second or more, and the cooling stop temperature is preferably 500 ° C. or less. When the cooling rate is less than 3 ° C./second or the cooling stop temperature exceeds 500 ° C., a large amount of ferrite is generated and the base material strength tends to deteriorate, and the nucleation driving force of bainite is low. In addition, the crystal grains become coarse and the toughness tends to deteriorate.
[0043]
In the steel sheet of the present invention, the tempering process may not be performed, but when the tempering process is performed, the tempering temperature is preferably set to 400 to 600 ° C. When the tempering temperature is less than 400 ° C., MA is not decomposed, and the toughness is not improved only by strength reduction, so that the effect of the tempering treatment cannot be ensured. On the other hand, when it exceeds 600 ° C., crystal grains grow and become coarse, and the base material toughness deteriorates.
[0044]
【Example】
Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.
[0045]
Steel having the chemical composition shown in Table 1 (steel type numbers 1 to 29) is melted by a normal melting method to form a slab, and then homogenized rolling, hot rolling, and tempering treatment are performed under the conditions shown in Tables 2 and 3. The steel plate for evaluation which consists of predetermined plate | board thickness (Evaluation steel plate No. 1-22 of Table 2, Evaluation steel plate No. 23-39 of Table 3) was manufactured.
[0046]
Thus obtained steel plate for evaluation No. The following measurements were performed for 1 to 39. The results are shown in Tables 4 and 5.
[0047]
[Base material properties]
Tensile test: JIS No. 4 specimen was taken from 1/4 thickness part of each steel plate for evaluation and tensile test (TS), yield strength at 0.2% elongation (0.2% yield strength) ) And tensile elongation (EL) were measured. Among these, regarding the tensile strength, 490 MPa ≦ TS <690 MPa was accepted.
[0048]
Impact test: A JIS No. 4 test piece was collected from a ¼ part of the thickness of each evaluation steel plate, and subjected to a Charpy impact test to determine the ductile fracture surface ratio, and the fracture surface transition temperature was calculated. The fracture surface transition temperature (vTrs) was -80 ° C. or lower.
[0049]
In addition, the following weldability test was implemented about what passed the preform | base_material specification.
[0050]
[Weldability]
HAZ toughness: After heating each steel plate for evaluation to 1400 ° C. and holding it for 5 seconds, heat cycle treatment of cooling from 800 ° C. to 500 ° C. in 500 seconds (HAZ when submerged arc welding with 70 kJ / mm heat input) (Corresponding to the heat history), a JIS No. 4 test piece was taken from a ¼ part of the plate thickness, and the Charpy impact test was conducted to measure the absorbed energy (vE- ₆₀ ). vE- ₆₀ ≧ 70 J was accepted.
[0051]
Resistance to weld cracking: Covered arc welding was performed at a heat input of 1.7 kJ / mm based on the y-type weld crack test method described in JIS Z 3158, and the root crack prevention preheating temperature was measured. 25 degrees C or less was set as the pass.
[0052]
[Bainite structure evaluation]
Etching with a 2% nital solution (2% nitric acid-ethanol solution) in a cross section parallel to the rolling direction at a thickness of 1/4 of each steel plate for evaluation, and a range of 200 μm × 150 μm using an optical microscope Was taken at 400 magnifications at 10 locations, and the photographs were subjected to image analysis by an image analyzer, and the bainite fraction was measured.
[0053]
[Average crystal grain size measurement]
The measurement was performed using an EBSP analyzer (manufactured by TexSEM Laboratories) and an FE-SEM (electrolytic emission scanning electron microscope) “XL30S-FEG” manufactured by Philips at the same place where the bainite structure evaluation was performed. The crystal grain size was determined with a boundary having an inclination of 10 degrees or more as a grain boundary. The measurement area was 100 μm, the measurement step was 0.4 μm, and measurement points with a confidence index (Confidence Index) indicating the reliability of the measurement direction of 0.1 or less were deleted from the analysis target. In addition, those having a crystal grain size of 1.2 μm or less were judged as measurement noise and excluded from the target for calculating the average value of the crystal grain size.
[0054]
[Table 1]

[0055]
[Table 2]

[0056]
[Table 3]

[0057]
[Table 4]

[0058]
[Table 5]

[0059]
From Tables 4 and 5, it can be considered as follows. Steel plate for evaluation No. 3, 4, 6, and 12 to 22 all use steel type numbers 1 to 12 in Table 1 with good chemical composition, and the structure is good as shown in Table 4, low temperature base material toughness, low temperature HAZ toughness The characteristics of various base materials such as and the weldability were good. On the other hand, the evaluation steel plate No. 1, 2, 5, 7 to 11, 23 to 39 are not suitable for the structure as shown in Table 4 because the manufacturing conditions are inappropriate as shown in Table 2 (Evaluation Steel Sheet Nos. 1 and 2) 5, 7 to 11), and the steel composition numbers 13 to 29 in Table 1 having an inappropriate chemical composition are used, so that the structure is unsuitable as shown in Table 5 (Evaluation Steel Plate Nos. 23 to 39). Have the defects.
[0060]
No. The steel plate for evaluation No. 1 is an example in which homogenization rolling treatment is not performed before hot rolling, No. 1 The steel plate for evaluation No. 2 is an example in which the heating temperature during the homogenization rolling treatment is low. No. The evaluation steel plate 5 is an example having a high tempering temperature. No. The steel plate for evaluation No. 7 is an example having a low cumulative rolling reduction from 1000 to 900 ° C. The steel plate for evaluation No. 8 is an example in which the cumulative rolling reduction from 900 to 700 ° C. is low. No. No. 9 steel plate for evaluation is an example where the cooling rate after hot rolling is slow, The steel plate for evaluation 10 is an example having a high cooling stop temperature after hot rolling. No. No. 11 steel plate is an example in which the heating temperature during hot rolling is high. These steel plates for evaluation have a large average crystal grain size and inferior low temperature base metal toughness. In the steel plates for evaluation of 9 and 10, the base material strength is also inferior.
[0061]
No. The steel plate for evaluation No. 23 is an example having a small amount of C, has a low bainite fraction, and a large average grain size. As a result, the base material strength and the low temperature base material toughness are inferior. No. The steel plate for evaluation of 24 is an example with a large amount of C, and the low-temperature HAZ toughness is inferior.
[0062]
No. The evaluation steel plate No. 25 is an example having a large amount of Si, and is inferior in low-temperature base material toughness and low-temperature HAZ toughness.
[0063]
No. The steel plate for evaluation No. 26 is an example having a small amount of Mn, has a low bainite fraction, and a large average grain size. As a result, the base material strength and the low temperature base material toughness are inferior.
[0064]
No. The steel plate for evaluation No. 27 is an example having a large amount of Mn, No. 27. The steel plates for evaluation of Nos. 28 and 29 are examples having high KP, No. No. 30 is an example in which the amount of Cr exceeds the preferable range and the KP is large. The evaluation steel plate 31 is an example having a large amount of Mo. These evaluation steel sheets have poor low-temperature HAZ toughness.
[0065]
No. The steel plate for evaluation No. 32 is an example having a large amount of Nb. The steel plate for evaluation of 33 is an example in which the V amount exceeds the preferable range. These steel plates for evaluation have a large average crystal grain size and are inferior in low temperature base metal toughness and low temperature HAZ toughness.
[0066]
No. The steel plate for evaluation of 34 is an example in which there are many Ni and Cu, and the low temperature HAZ toughness is inferior.
[0067]
No. The steel sheet for evaluation No. 35 is an example in which the B content is small and the bainite fraction is low, and the base metal strength is inferior. No. The steel plate for evaluation No. 36 is an example having a large B amount and a large average crystal grain size, and is inferior in low-temperature base material toughness and low-temperature HAZ toughness.
[0068]
No. The steel plate for evaluation No. 37 is an example with a large amount of N, No. 37. The evaluation steel plate No. 39 is an example having a large amount of Ti, and these evaluation steel plates have a large average crystal grain size and are inferior in low-temperature base material toughness and low-temperature HAZ toughness. No. The steel plate for evaluation No. 38 is an example having a small amount of Ti, and the low-temperature HAZ toughness is inferior.
[0069]
【The invention's effect】
The present invention is configured as described above, and in addition to a specific chemical composition (particularly KP), by setting the bainite fraction and the average crystal grain size of the steel structure to a specific range, the low temperature base material toughness and the low temperature It was possible to provide a 490 MPa grade or higher steel plate having excellent HAZ toughness (high heat input HAZ toughness). The steel plate of the present invention is suitable for use in a structure (welded structure) exposed to a low temperature environment such as a ship or an offshore structure.

Claims (14)

C: 0.010 to 0.070% (meaning mass%, the same applies hereinafter), Si: 0.8% or less, Mn: 1.3 to 1.9%, Nb: 0.032% or less (0% Ti): 0.005-0.10%, B: 0.0006-0.0050%, N: 0.0020-0.010%, Al: 0.20% or less, P: 0.020% Hereinafter, S: 0.010% or less, the balance: made of steel satisfying Fe and inevitable impurities ,
While satisfying KP <2.4,
Is 50 vol% or more bainite steel structure, and an average grain size of Ri der less 8 [mu] m, low temperature preform and the balance ferrite, martensite, or a composite structure der Rukoto of martensite and austenite, High toughness steel sheet with excellent toughness and low temperature HAZ toughness .
However,
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo]
<< In formula, [] means content (mass%) of each element. >> C: The high toughness steel plate according to claim 1 which is 0.010 to 0.055%. Furthermore, Ni: 2.0% or less, or Cu: 2.0% or less, The high toughness steel plate of Claim 1 or 2 . The high toughness steel sheet according to any one of claims 1 to 3, further comprising Cr: 1.0% or less, Mo: 0.30% or less, or V: 0.10% or less. Furthermore, Ca: 0.0050% or less is contained, The high-toughness steel plate in any one of Claims 1-4 . Furthermore, Mg: 0.005% or less, REM: 0.02% or less, or Zr: 0.050% or less, The high toughness steel plate in any one of Claims 1-5 . The high toughness steel sheet according to any one of claims 1 to 6 , wherein the thickness is 50 mm or more. C: 0.010 to 0.070%, Si: 0.8% or less, Mn: 1.0 to 1.9%, Nb: 0.032% or less (including 0%), Ti: 0.005 0.10%, B: 0.0006 to 0.0050%, N: 0.0020 to 0.010%, Al: 0.20% or less, P: 0.020% or less, S: 0.010% or less , Balance: using steel satisfying Fe and inevitable impurities and satisfying KP <2.4,
A homogenizing rolling step of heating at a temperature of 1150 to 1350 ° C .;
After heating at a temperature of 1050 to 1150 ° C., applying a reduction of a cumulative reduction ratio of 40% or more at a temperature of 1000 to 900 ° C., and applying a reduction of a cumulative reduction ratio of 60% or more at a temperature of 900 to 700 ° C. A hot rolling step of ending rolling at a temperature of 700 ° C .;
A cooling step of cooling to a temperature of 500 ° C. or lower at a cooling rate of 3 ° C./second or higher;
A method for producing a high toughness steel sheet, characterized in that
However,
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo]
<< In formula, [] means content (mass%) of each element. >> The manufacturing method of the high toughness steel plate of Claim 8 which performs tempering at the temperature of 400-600 degreeC after the said cooling process further. The method for producing a high toughness steel plate according to claim 8 or 9, wherein the steel further contains Ni: 2.0% or less, or Cu: 2.0% or less. The high toughness according to any one of claims 8 to 10, wherein the steel further contains Cr: 1.0% or less, Mo: 0.30% or less, or V: 0.10% or less. A method of manufacturing a steel sheet. The method for producing a high toughness steel plate according to any one of claims 8 to 11, wherein the steel further contains Ca: 0.0050% or less. The steel further contains Mg: 0.005% or less, REM: 0.02% or less, or Zr: 0.050% or less. The manufacturing method of the high toughness steel plate as described in a crab. The method for producing a high toughness steel plate according to any one of claims 8 to 13, wherein the thickness is 50 mm or more.