JP6024928B2 - Steel plates for marine, marine structures and hydraulic iron pipes with excellent brittle crack propagation stopping properties and methods for producing the same - Google Patents

Description

The present invention is used for ships, offshore structures, hydraulic iron pipes, etc., and has a tensile strength of 570 MPa or more and 720 MPa or less, and is excellent in brittle crack propagation stop characteristics, particularly in the non-ductile transition temperature (T _NDT ) in the NRL drop weight test. The present invention relates to a thick steel plate and a manufacturing method thereof. In addition, the “thick steel plate” of the present invention refers to a hot rolled steel plate having a thickness of 6 mm or more and 100 mm or less.

  Steel sheets used for ships, offshore structures, hydraulic iron pipes and the like are required to have good weldability in addition to excellent strength characteristics and low temperature toughness. Particularly in recent years, due to the expansion of energy demand and the like, it has come to be used in a lower temperature environment than before, and development of a material having excellent characteristics has been strongly desired.

  The Charpy impact test is generally used as the evaluation standard for the low temperature toughness, but the NRL drop weight test is specified as a standard for further evaluating the brittle crack propagation stop characteristic in ASTM. Particularly in recent years, as the service temperature is lowered, the required temperature in the NRL drop weight test has been lowered, and the demand for improving the NRL drop weight characteristic for steel sheets has also increased.

  By the way, in order to improve the low-temperature toughness, it has been conventionally known that addition of Ni is effective. However, since Ni is an expensive element and causes an increase in raw material cost, it is desired to satisfy the necessary NRL drop weight characteristics while keeping the amount of Ni added as low as possible.

  As a tempered high-strength steel sheet having a tensile strength TS of 570 MPa or more and 720 MPa or less and not containing a large amount of Ni and having excellent NRL drop weight characteristics, for example, Patent Document 1 discloses C: 0.08. ~ 0.16 mass%, Ni: 0.15 to 0.60 mass%, Nb and B are essential additive elements, and the microstructure is a lower bainite structure, thereby improving the NRL drop weight characteristics 40 mm to 80 mm A steel sheet is disclosed.

  In Patent Document 2, C: 0.04 to 0.08 mass%, Ni: 0.05 to 0.60 mass%, a tempered 600 MPa grade steel plate substantially free of Nb and B, A steel sheet having a thickness of 80 mm or less is disclosed in which the drop weight characteristic after post-weld heat treatment (PWHT) is improved by using tempered bainite and / or tempered martensite as the structure.

  Further, Patent Document 3 includes C: 0.1 to 0.16 mass%, Ni: 0.6 mass% or less, and B: 0.0005 to 0.002 mass%, and F = 9.4 × [Mo]. The F value consisting of + 8.1 × [V] + 4.7 × [Cr] is 3.20 or more and 4.50 or less, and the average equivalent circle diameter of crystal grains surrounded by a large-angle grain boundary of 15 ° or more is 4 μm or less. A steel sheet having a thickness of 80 mm or more with improved drop weight characteristics by using a tempered bainite structure is disclosed.

  Patent Document 4 contains C: 0.03 to 0.150 mass%, Ni: 2 mass% or less, and has a microstructure with an area ratio of bainite of 90% or more and an average value of bainite lath width. A steel sheet having a thickness of 50 mm or more with improved drop weight characteristics is disclosed by setting the maximum equivalent circle diameter of island martensite in bainite to 3.5 μm or less and 3.0 μm or less.

Japanese Patent Application Laid-Open No. 09-194989 JP 2011-001620 A JP 2011-047032 A JP 2011-179106 A

However, in the techniques disclosed in Patent Documents 1 and 2, the drop weight characteristic T _NDT described is about −30 ° C. to −50 ° C., and the recent demand for NRL drop weight characteristics (T _NDT) ≦ −55 ° C.) cannot be satisfied. In addition, the technique disclosed in Patent Document 3 is intended for thick materials with a plate thickness of 80 mm or more, and in thick materials with a slow cooling rate during quenching, good drop weight characteristics can be obtained. Since the amount is as high as 0.10 mass% or more, when it is applied to a steel plate having a thickness of about 40 to 60 mm, it becomes a martensite-based structure, and there is a concern that the drop weight characteristic may be deteriorated. In addition, since the technique disclosed in Patent Document 4 is intended for a steel sheet having a thickness of 50 mm or more, particularly 100 mm, the microstructure of the steel sheet is a structure mainly composed of an upper bainite structure including island martensite. In order to reduce the island-like martensite in the upper bainite, it is essential to perform tempering twice, so that there is a problem that the manufacturing cost is high.

In recent years, by lowering the design temperature, as T _NDT of NRL drop weight test, have come to consider the safety margin is -55 ° C. or less is required. Therefore, in the prior art, a tempering type 600 MPa class subjected to quenching and tempering treatment, and a steel plate having a thickness of 100 mm or less, a brittle crack propagation stop characteristic, that is, NRL without requiring a large amount of Ni addition. the it is difficult to the T _NDT of drop weight test as -55 ° C. or less was circumstances.

The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof is to add a large amount of Ni, and have a tensile strength TS of 570 MPa or more and 720 MPa or less, and an NRL drop weight test. In addition to providing a thick steel plate having a _TNDT of −55 ° C. or less and a plate thickness of 100 mm or less, an advantageous manufacturing method thereof is proposed.

The inventors have intensively studied to solve the above problems. As a result, when a steel material (slab) having a predetermined component composition is hot-rolled to 100 mm or less under appropriate conditions and then quenched and tempered to produce a thick steel plate, the tensile strength TS after quenching Is controlled to 850 MPa or more and 1100 MPa or less, then tempered at an appropriate temperature, and the microstructure is tempered lower bainite and tempered martensite, so that the tensile strength TS is 570 MPa or more and 720 MPa or less, and the NRL drop weight test As a result, it was found that a thick steel plate excellent in brittle crack propagation stopping characteristics having a _TNDT of −55 ° C. or lower can be obtained, and the present invention has been developed.

Based on the above findings, the present invention provides C: 0.05 to 0.15 mass%, Si: 0.05 to 0.60 mass%, Mn: 0.8 to 1.8 mass%, P: 0.020 mass% or less, S : 0.005 mass% or less, Al: 0.005 to 0.080 mass%, N: 0.0005 to 0.0050 mass%, Cu: 0.50 mass% or less, Ni: 1.50 mass% or less, Cr: 0.50 mass% or less, Mo: 0.40 mass% or less, V: 0.07 mass% or less, and B: 0.0003 to 0.0020 mass%, or one or more selected from the balance, and the balance Has a component composition consisting of Fe and inevitable impurities, the following formula (1):
C _eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
The C equivalent C _eq (mass%) defined by the formula is 0.36 to 0.55, and the following formula (2):
DI ^* = 8.2 × (C) ^0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%)
The DI ^* value defined by the above is 65 or more, and the microstructure at the ¼ thickness position is composed of tempered lower bainite and tempered martensite structure, and the tensile strength is 570 MPa or more and 720 MPa or less, marine where T _NDT of NRL drop weight test at a thickness 1/4 position, characterized in that at -55 ° C. or less, a steel plate for use and penstock marine structure.

  In addition to the above component composition, the thick steel plate of the present invention further includes Nb: 0.030 mass% or less, Ti: 0.030 mass% or less, Ca: 0.0040 mass% or less, REM: 0.010 mass% or less, W: It contains one or more selected from 0.40 mass% or less, Co: 0.50 mass% or less, Mg: 0.005 mass% or less, and Zr: 0.02 mass% or less.

  In the thick steel plate of the present invention, the average equivalent circle diameter of crystal grains surrounded by a large-angle grain boundary having a misorientation angle of 15 degrees or more at a 1/4 thickness position is 5 μm or less, and prior austenite grains The average particle size of the material is 30 μm or less, and the variation σ is 15 μm or less.

Further, the present invention is a slab formed from a component composition according to the hot-rolled, then, in the thickness 1 / 4t position in the temperature range up to Ac ₁ transformation point to Ac ₃ transformation point at the time of re-heating After heating at a heating rate of 1 to 30 ° C./min and reheating to a temperature of Ac ₃ transformation point to (Ac ₃ transformation point + 70 ° C.), quenching is performed at least once, and then 550 to 700 ° C. provide Reinforced tempered at a temperature, the microstructure in the sheet thickness 1/4 position consists tempered lower bainite and tempered martensite structure, and a tensile strength of more than 570 MPa, 720 MPa or less, the sheet thickness 1/4 position T _NDT of NRL drop weight test in proposes a marine manufacturing method of offshore structures and for penstock for steel plate, characterized in that to obtain the following steel plate -55 ° C..

  The method for producing a thick steel plate according to the present invention is characterized in that the tensile strength as-quenched at a ¼ position of the plate thickness is 850 MPa to 1100 MPa.

According to the present invention, the tensile strength of more than 570 MPa, which has the following high strength 720 MPa, for _{vessels T NDT} in NRL drop weight test is excellent in brittle crack propagation stop characteristics of -55 ° C. or less, and for marine structures It is possible to stably supply a thick steel plate for a hydraulic iron pipe.

It is a diagram schematically showing the relationship between the NRL drop weight characteristic T _NDT after tensile strength and tempering after quenching in microalloyed type 600MPa class steel. It is the figure which showed the result of the Example on FIG.

First, the thick steel plate that is the development target of the present invention will be described.
The present invention corresponds to a tempered 600 MPa class steel, has a tensile strength TS of 570 MPa or more and 720 MPa or less, and corresponds to the recent lowering of the design temperature required by ship class standards, etc. The goal is to develop a thick steel plate having low-temperature toughness characteristics (brittle crack propagation stopping characteristics) with a non-ductile transition temperature T _NDT of −55 ° C. or less. Here, the above-mentioned _{T NDT} in the present invention, non-ductile transition temperature in NRL drop weight test as specified in ASTM E208 (NDT Temperature: nil-ductility transition temperature) means that.

Next, the experiment that led to the development of the present invention will be described.
In order to develop a steel having the above-described high strength and excellent brittle crack propagation stopping characteristics, the inventors further added a steel containing C: 0.04 to 0.20 mass%, Ni: 1.50 mass% or less, , Si, Mn, Cu, Cr, Mo and V are added, and the following formula (1):
C _eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
C equivalent C _eq (mass%) defined in the above is melted in a laboratory with steel having various component compositions in the range of 0.35 to 0.55 mass%, and the thickness is 25 mm by hot rolling. Then, the steel plate was reheated and quenched at various cooling rates of 2 ° C./s or higher. The quenched steel sheet was subjected to a tensile test to measure the tensile strength TS, and the microstructure was observed using an optical microscope and a scanning electron microscope.
Subsequently, the quenched steel plate was tempered at a temperature of 550 to 700 ° C. to obtain a tempered 600 MPa class thick steel plate having a tensile strength TS of 570 to 720 MPa after tempering.
About the thick steel plate thus obtained, in accordance with ASTM standard A370, a test piece is taken in the C direction, a Charpy impact test is performed in accordance with A370, the fracture surface transition temperature vTrs is measured, and in accordance with ASTM E208. Then, a P-3 test piece (thickness 16 mm × width 50 mm × length 130 mm) was sampled in the C direction, a drop weight test was performed, and a non-ductile transition temperature T _NDT was obtained.

Figure 1 differs from that of the tensile strength TS and microstructure after quenching, the relationship between T _NDT in drop weight test after tempering is shown schematically. From this figure, the hardenability is increased to a tensile strength TS after quenching of 850 MPa or more and 1100 MPa or less, and the steel microstructure is changed to lower bainite and martensite, and then tempered to obtain a tensile strength TS of 570 MPa or more and 720 MPa or less. It can be seen that only in this case, the non-ductile transition temperature T _NDT in the drop weight test after tempering can be set to −55 ° C. or lower.
The present invention has been developed based on the above novel findings.

Next, the reason which limits the component composition of the thick steel plate of this invention is demonstrated.
C: 0.05-0.15 mass%
C is an essential element for ensuring the hardenability and strength of the steel. The component composition of C _eq is 0.50 or less, and in order to set the tensile strength after quenching to 850 MPa or more, 0 .05 mass% or more must be contained. On the other hand, if added in excess of 0.15 mass%, the toughness of the base metal and the weld heat-affected zone and the weldability are reduced. Therefore, the C content is in the range of 0.05 to 0.15 mass%. Preferably it is the range of 0.055-0.12 mass%, More preferably, it is the range of 0.075-0.10 mass%.

Si: 0.05-0.60 mass%
Si is an element added as a deoxidizer for steel. It is also an effective element for increasing the strength of steel by solid solution strengthening. In order to acquire said effect, it is necessary to contain 0.05 mass% or more. On the other hand, if added over 0.60 mass%, the toughness of the weld heat affected zone decreases. Therefore, the Si content is in the range of 0.05 to 0.60 mass%. Preferably it is the range of 0.10-0.40 mass%.

Mn: 0.8 to 1.8 mass%
Mn is an element added as a deoxidizer for steel. It is also an element effective in improving the hardenability of steel and increasing its strength. In order to acquire the said effect, it is necessary to make it contain 0.8 mass% or more. On the other hand, if added over 1.8 mass%, the weldability and the toughness of the heat affected zone are reduced. Therefore, the Mn content is in the range of 0.8 to 1.8 mass%. Preferably it is the range of 1.2-1.6 mass%.

P: 0.020 mass% or less P is inevitably mixed in steel and segregates in the prior austenite grain boundaries during cooling after tempering or in the annealing process of stress relief annealing (PWHT) after welding. It promotes interfacial embrittlement and reduces the falling weight characteristics. Therefore, it is desirable to reduce the content of P as much as possible, but if it is 0.020 mass% or less, grain boundary embrittlement can be prevented. Therefore, in the present invention, the P content is limited to 0.020 mass% or less. Preferably it is 0.015 mass% or less. More preferably, it is 0.010 mass% or less.

S: 0.005 mass% or less S is an element that is inevitably mixed in steel and exists as sulfide inclusions such as MnS in the steel, and is an element that lowers toughness. Therefore, it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is limited to 0.005 mass% or less in order to ensure the falling weight characteristic. Preferably it is 0.003 mass% or less, More preferably, it is 0.002 mass% or less, More preferably, it is 0.001 mass% or less.
It is.

Al: 0.005-0.080 mass%
Al is an element added as a deoxidizing element, and is a useful element that contributes to improving toughness through refinement of crystal grains during quenching heating with AlN. In order to express these effects, it is necessary to contain 0.005 mass% or more. On the other hand, if it is added in excess of 0.080 mass%, it causes a problem that the base metal toughness is reduced due to the coarsening of the crystal grains due to the coarsening of AlN and the toughness of the welded hot zone is reduced. Therefore, the Al content is in the range of 0.005 to 0.080 mass%. Preferably it is the range of 0.020-0.060mass%.

N: 0.0005 to 0.0050 mass%
N contributes effectively to improving the toughness of the steel by forming Al and AlN and refining the austenite grains during quenching heating. In order to acquire the said effect, it is necessary to contain 0.0005 mass% or more. However, if added over 0.0050 mass%, the toughness of the welded portion is reduced. Therefore, the N content is in the range of 0.0005 to 0.0050 mass%. Preferably it is the range of 0.0010-0.0040 mass%.

In addition to the above components, the thick steel plate of the present invention needs to contain one or more selected from the following Cu, Ni, Cr, Mo, V and B.
Cu: 0.50 mass% or less Cu is an element useful for increasing the strength of steel by solid solution strengthening. However, if added over 0.50 mass%, Cu cracks may occur during hot rolling. Therefore, the Cu content is 0.50 mass% or less. Preferably it is the range of 0.01-0.40 mass%.

Ni: 1.50 mass% or less Ni, as well as Cu, is an element useful as a solid solution strengthening element and an element useful for improving low-temperature toughness. However, if it is added in excess of 1.50 mass%, the raw material cost is increased, or slab cracking is caused to hinder productivity. Therefore, the Ni content is 1.50 mass% or less. Preferably it is 0.01-1.20 mass%, More preferably, it is the range of 0.01-1.00 mass%.

Cr: 0.50 mass% or less Cr is an element useful for improving the hardenability and increasing the strength of the steel, and for ensuring the strength after tempering or PWHT. However, if added over 0.50 mass%, the weldability is lowered and the toughness after PWHT is significantly lowered. Therefore, the Cr content is set to a range of 0.50 mass% or less. Preferably it is the range of 0.01-0.30 mass%.

Mo: 0.40 mass% or less Mo is an element useful for improving the hardenability and increasing the strength of the steel and ensuring toughness. Moreover, like Cr, it is an element useful for ensuring the strength after tempering or PWHT. However, if added over 0.40 mass%, the weldability is lowered and the raw material cost is increased. Therefore, the Mo content is set to a range of 0.40 mass% or less. Preferably it is the range of 0.01-0.30 mass%.

V: 0.07 mass% or less V is an element useful for improving the hardenability of steel, forming carbon and nitrides with C, N, and ensuring strength after tempering and PWHT. However, if added over 0.07 mass%, the weldability is lowered and the base material toughness is lowered due to carbonitride precipitation. Therefore, the content of V is set to 0.07 mass%. Preferably it is the range of 0.003-0.06 mass%.

B: 0.0003 to 0.0020 mass%
B is an element that enhances the hardenability with a very small amount of addition and makes the microstructure lower bainite or martensite structure and contributes effectively to improving the strength of steel. In order to obtain the effect of improving the hardenability, addition of 0.0003 mass% or more is required. On the other hand, even if added over 0.0020 mass%, the above effect is only saturated. Therefore, B is added in the range of 0.0003 to 0.0020 mass%. Preferably it is the range of 0.0005-0.0015 mass%.

Moreover, in addition to satisfy | filling said component composition, the thick steel plate of this invention has the following (1) formula;
C _eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
The C equivalent C _eq (mass%) defined by the above is required to be in the range of 0.36 to 0.55 mass%.
When C _eq is 0.36 mass% or less, hardenability at the time of quenching cannot be secured and it becomes difficult to obtain a strength of 600 MPa after tempering. On the other hand, if it exceeds 0.55 mass%, the toughness of the base metal and the weld heat affected zone will be reduced. Preferably it is the range of 0.40-0.53 mass%.

Moreover, in addition to satisfy | filling the said component composition, the thick steel plate of this invention has the following (2) formula;
DI ^* = 8.2 × (C) ^0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%)
It is necessary that the DI ^* value defined by is 65 or more.
Here, the above DI ^* value is an index representing _{hardenability} like the C equivalent C _eq, and the DI formula described in the literature (for example, heat treatment; vol. 27, No. 3, p. 136). Was used by fitting the coefficient from the experimental result. If the DI ^* value is less than 65, the hardenability cannot be ensured, the tensile strength after quenching does not become 850 MPa or more, and the desired lower bainite and martensite structure cannot be obtained.

In the thick steel plate of the present invention, the balance other than the essential components described above is Fe and inevitable impurities, but in order to increase the strength and low temperature toughness of the steel, in addition to the above components, Nb, Ti, Ca, REM , W, Co, Mg, and Zr can be included.
Nb: 0.030 mass% or less Nb, like V, is an element that forms carbonitrides with C and N and increases the strength of steel. It is also an element that contributes effectively to improving toughness through austenite grain refinement during quenching. However, when PWHT is applied, embrittlement due to precipitation of Nb carbonitride is large, and particularly the toughness of the weld heat-affected zone is markedly reduced. Therefore, it is preferable to add 0.030 mass% as the upper limit.

Ti: 0.030 mass% or less Ti is an element that forms carbonitride with C and N, and is effective in improving toughness through refinement of austenite grains, and therefore can be added. However, if added over 0.030 mass%, precipitation embrittlement occurs. Therefore, it is preferable to add 0.030 mass% or less.

Ca: 0.0040 mass% or less Ca controls the form of sulfide (MnS), improves the low-temperature toughness and anisotropy of the base material, improves resistance to hydrogen-induced cracking, and reheat cracking (occurs during stress relief annealing) It is a useful element that is effective in suppressing cracking. However, addition exceeding 0.0040 mass% produces Ca oxysulfide (inclusions), and harms low temperature toughness and cleanliness. Therefore, the Ca content is preferably added in the range of 0.0040 mass% or less.

REM: 0.010 mass%
REM, like Ca, is an element effective for controlling the form of sulfide (MnS), improving low-temperature toughness and anisotropy of the base material, improving hydrogen-induced crack resistance, and suppressing reheat cracking. However, if added over 0.010 mass%, the low temperature toughness and cleanliness are reduced. Therefore, the amount of REM added is preferably 0.010 mass% or less.

W: 0.40 mass% or less W is an element that improves the hardenability and increases the strength of the steel, but if added over 0.40 mass%, the base metal toughness is reduced. Therefore, the amount of W added is preferably 0.40 mass% or less.

Co: 0.50 mass% or less Co is an element that improves the hardenability and increases the strength of the steel, but if added in excess of 0.50 mass%, the base metal toughness is reduced. Therefore, the amount of Co added is preferably 0.50 mass% or less.

Mg: 0.005 mass% or less Mg is an element that contributes to improving the toughness of the weld heat-affected zone by dispersing the oxide. However, if added over 0.005 mass%, the base metal toughness and welding heat are conversely added. Reduces affected area toughness. Therefore, the amount of Mg added is preferably 0.005 mass% or less.

Zr: 0.02 mass% or less Zr is an element that contributes to the improvement of toughness of the weld heat affected zone due to the dispersion of oxides, but if added over 0.02 mass%, the base metal toughness and welding heat Reduces affected area toughness. Therefore, the amount of Zr added is preferably 0.02 mass% or less.

Next, the microstructure after quenching and tempering of the thick steel plate of the present invention will be described.
The microstructure after quenching and tempering of the thick steel plate of the present invention needs to be tempered lower bainite and tempered martensite. The reason for the microstructure is to refine the effective crystal grain size (fracture surface unit of cleavage fracture) that becomes resistance to crack propagation in brittle crack propagation in order to improve low-temperature toughness, especially NRL drop weight characteristics. Because it is important. In addition, the structure other than the tempered lower bainite and the tempered martensite may be mixed so long as the effect of the present invention is not impaired, specifically, 10 vol% or less. Moreover, in the thick steel plate of the present invention after tempering, island-like martensite is decomposed into cementite, and thus basically does not exist, but it is acceptable if it is 0.5 vol% or less.

Further, the microstructure after quenching and tempering of the thick steel plate of the present invention has an average equivalent circle diameter of 5 μm or less of crystal grains surrounded by large-angle grain boundaries of 15 degrees or more as measured by an electron backscattering pattern (EBSP). It is preferable that Here, the above-mentioned large-angle grain boundary of 15 degrees or more is based on the orientation mapping measured by EBSP, and the grain boundary angle between adjacent crystal grains is obtained, and the grain boundary whose value is 15 degrees or more is obtained. means. The smaller the crystal grain surrounded by a large-angle grain boundary of 15 degrees or more measured by EBSP, the more the stress concentration at the crystal grain boundary can be reduced, so the resistance to fracture increases and the low-temperature toughness and drop weight characteristics improve. To do. That is, when the average grain size of the crystal grains surrounded by the large-angle grain boundaries of 15 degrees or more exceeds 5 μm, breakage occurs starting from coarse grains and the toughness is reduced. The falling weight characteristic (T _NDT ≦ −55 ° C.) is obtained.

  Further, the prior austenite particle size needs to have an average particle size of 30 μm or less and a variation σ of 15 μm or less. By making the prior austenite grain size smaller than 30 μm, it is possible to refine the crystal grain size surrounded by large-angle grain boundaries with a misorientation angle measured by EBSP of the lower bainite and martensite structure of 15 degrees or more, Drop weight characteristics are improved. Furthermore, by reducing the coarse grain by setting the variation σ of the prior austenite grain size to 15 μm or less, it is possible to improve the falling weight characteristic more stably. However, if the prior austenite grain size exceeds 30 μm, or the variation exceeds 15 μm, the lower bainite and martensite transformed from coarse austenite grains become coarse, and the effective crystal grain size becomes coarse accordingly. , The falling weight characteristic is reduced.

Here, the equivalent circle diameter of the crystal grain surrounded by the grain boundary means the equivalent circle diameter of the crystal grain obtained by tracing the grain boundary and analyzing the image.
In addition, the prior austenite grain size refers to the equivalent circle diameter obtained by tracing the grain boundary and analyzing the image after imaging the structure where the grain boundary appeared using an appropriate microscope using an optical microscope, The crystal grain distribution is statistically analyzed and calculated by image analysis. The variation σ means a standard deviation.

Next, the manufacturing method of the thick steel plate of this invention is demonstrated.
The steel plate of the present invention is a steel material (slab) that has been prepared by melting steel whose component composition has been adjusted to the above-mentioned range, continuously casting, or ingoting, and then rolling it into a steel material (slab), and then reheating, Hot rolled to obtain a thick steel plate having a thickness of 6 to 100 mm, and then subjected to quenching-tempering treatment.
Here, the said hot rolling should just be performed according to a conventional method, and there is no restriction | limiting in particular. For example, after reheating the slab to a temperature of 1000 to 1200 ° C., normal hot rolling may be performed. The cooling after rolling need not be particularly limited, and may be air-cooled.

The thick steel plate obtained by the above hot rolling is then heated at a heating rate of 1 to 30 ° C./min in the temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point at the ¼ thickness position, and Ac ₃ after reheating to a temperature range of transformation point ~ (Ac ₃ transformation point + 70 ° C.), is a hardening process for hardening must be applied more than once.
The reason why the heating rate of the two-phase region from the Ac ₁ transformation point to the Ac ₃ transformation point is 30 ° C./min or less is that when the heating rate exceeds 30 ° C./min, reverse transformation to austenite is performed. This is because sometimes the austenite grain size is likely to vary, leading to a reduction in the base material toughness. On the other hand, when the temperature is less than 1 ° C./min, the time required for temperature increase becomes long, which is disadvantageous in terms of productivity and manufacturing cost.
If the quenching temperature is less than the Ac ₃ transformation point, a two-phase region of (austenite + ferrite) is obtained during reheating, and the austenite grain size becomes small and quenching becomes insufficient. I can't. On the other hand, if the heating exceeds (Ac ₃ transformation point + 70 ° C.), the austenite grain size becomes coarse, the toughness of the base material decreases significantly, and the drop weight characteristic also decreases.

Incidentally, the Ac ₁ transformation point, the temperature of the Ac ₃ transformation point can be determined by actually measuring it, it is possible to use a value determined by the following equation.
Ac ₁ (° C.) = 751-27 × C + 18 × Si-12 × Mn-169 × Al-23 × Cu-23 × Ni + 24 × Cr + 23 × Mo-40 × V + 233 × Nb-6 × Ti-895 × B
Ac ₃ (° C.) = 937-477 × C + 56 × Si-20 × Mn + 198 × Al-16 × Cu-27 × Ni-5 × Cr + 38 × Mo + 125 × V-19 × Nb + 136 × Ti + 3315 × B
(However, each element symbol in the above formula is the content of each element (mass%))

Moreover, although the quenching conditions after reheating to the said temperature change with board thickness, it is preferable to water-cool with a cooling rate of 2 degrees C / s or more. When the cooling rate is less than 2 ° C./s, hardenability cannot be ensured, the desired lower bainite and martensite structure cannot be obtained, and the drop weight characteristic is lowered.
Furthermore, the number of times of quenching is usually sufficient, but it is performed twice or more, and by repeating the transformation to austenite and transformation to ferrite, the austenite grain size at the time of quenching is refined and adjusted. Since it granulates, since the effect which improves a base material toughness can be acquired, it is preferable.

  The steel plate subjected to the above quenching treatment needs to be subjected to a tempering treatment at a relatively high temperature of 550 to 700 ° C. in consideration of minimizing the strength reduction when PWHT is performed thereafter. . When the tempering temperature is less than 550 ° C., the strength is greatly reduced when PWHT is performed at a temperature of 550 ° C. or higher, and a desired strength (TS: 570 MPa or higher, 720 MPa or lower) cannot be secured stably. On the other hand, when the temperature exceeds 700 ° C., it is difficult to ensure the desired strength after tempering. The cooling after the tempering process may be air cooling.

  Here, what is important in the present invention is to control the tensile strength at ¼ position of the thickness of the as-quenched steel sheet to 850 MPa or more and 1100 MPa or less. In a steel sheet having a thickness of 100 mm or less having the above-described component composition, the structure at the quarter thickness position after quenching is the lower bainite and martensite structure, and the drop weight characteristics are improved through refinement of the effective crystal grain size. Therefore, it is necessary to control the tensile strength after quenching to 850 MPa or more and 1100 MPa or less. If it is less than 850 MPa, hardenability is inadequate and structures, such as an upper bainite and a ferrite, will be mixed, and base material toughness, especially a drop weight characteristic will fall large. On the other hand, when the pressure exceeds 1100 MPa, the drop weight characteristic deteriorates. The tensile strength as quenched is more preferably in the range of 900 to 1050 MPa.

  In addition, although this invention makes the object of application the thick steel plate of 100 mm or less in thickness, it is 30-80 mm which is the main plate thickness of the thick steel plate especially used for a ship, a marine structure, a hydraulic iron pipe, etc. In a thick steel plate having a thickness of 40 to 70 mm, an excellent effect is obtained.

No. having various component compositions shown in Table 1. Steels 1 to 43 were melted and continuously cast into a steel material (slab) having a thickness of 310 mm, then hot rolled (thick plate rolling) under various conditions shown in Table 2, and then air-cooled. Thick steel plates (Nos. 1 to 48) having the thicknesses shown in Table 2 were used.
Next, the thick steel plate was reheated in a heating furnace under various conditions, which are also shown in Table 2, and then water-cooled and quenched, and then reheated in a heating furnace and then air-cooled.

The thick steel plate thus obtained was subjected to the following test.
<Microstructure>
After polishing the cross section of the sample taken from the rolling direction (L direction) of the thick steel plate and corroding with the nital solution, at the position of the plate thickness 1/4, the structure is observed using an optical microscope and a scanning electron microscope, A metallographic phase was identified.
In addition, from the orientation difference mapping measured by EBSP at the same plate thickness 1/4 position of the above sample, the grain boundary of the crystal grain surrounded by the grain boundary where the grain boundary orientation difference between adjacent crystal grains is 15 degrees or more is traced. The equivalent circle diameter was determined by image analysis, and the average crystal grain size was calculated.
<Tensile test>
Tensile test in which the diameter specified in ASTM A370 is 12.7 mm and GL is 50.8 mm so that the tensile direction is perpendicular to the rolling direction (C direction) from 1/4 position of the thick steel plate and the quenched steel plate. Pieces were collected and subjected to a tensile test according to A370, and the yield stress YS and the tensile strength TS were measured.
<Evaluation of toughness>
A 2 mm V notch Charpy impact test piece specified in ASTM A370 is sampled so that it is perpendicular to the rolling direction (C direction) from the thickness 1/4 position of the steel plate, and a Charpy impact test is performed in accordance with A370. The fracture surface transition temperature (vTrs) was obtained.
<Drop weight characteristics>
A P-3 specimen (thickness 16 mm x width 50 mm x length 130 mm) is taken from the position of 1/4 of the thickness of the steel plate so as to be perpendicular to the rolling direction (C direction) in accordance with ASTM E208. and performs drop weight test was determined no ductile transition temperature _{T NDT.}

The results of the above measurements are also shown in Table 2. From these results, No. 1 suitable for the present invention. Steel plate of 1 to 24 is tempered tensile strength of after turning at least 850 MPa, or less 1100 MPa, tensile strength TS after tempering is in the range of 570～720MPa, and the _{T NDT} in drop weight test - It turns out that it has the outstanding toughness characteristic of 55 degrees C or less.
On the other hand, No. which does not satisfy the conditions of the present invention. Steel plate of 25 to 48 may be any one or more characteristics of T _NDT in tensile strength TS and drop weight test, it does not satisfy the above condition.
For reference, the results of plotting the results of the above-described invention examples and comparative examples on the relationship diagram between the tensile strength after quenching shown in FIG. 1 and the non-ductile transition temperature T _NDT after tempering are shown in FIG. Indicated.

Claims (5)

C: 0.05-0.15 mass%,
Si: 0.05-0.60 mass%,
Mn: 0.8 to 1.8 mass%,
P: 0.020 mass% or less,
S: 0.005 mass% or less,
Al: 0.005-0.080 mass%,
N: 0.0005 to 0.0050 mass%,
Cu: 0.50 mass% or less,
Ni: 1.50 mass% or less,
Cr: 0.50 mass% or less,
Mo: 0.40 mass% or less,
V: 0.07 mass% or less and B: One or more selected from 0.0003 to 0.0020 mass%, with the balance being composed of Fe and inevitable impurities,
C equivalent C _eq (mass%) defined by the following formula (1) is 0.36 to 0.55,
The DI ^* value defined by the following formula (2) is 65 or more, and
Microstructure in the sheet thickness 1/4 position consists tempered lower bainite and tempered martensite structure, and a tensile strength of more than 570 MPa, below 720 MPa, the _{T NDT} of NRL drop weight test in the sheet thickness 1/4 position - Thick steel plates for ships, offshore structures and hydraulic iron pipes characterized by being 55 ° C or lower.
C _eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
DI ^* = 8.2 × (C) ^0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%) In addition to the above component composition, Nb: 0.030 mass% or less, Ti: 0.030 mass% or less, Ca: 0.0040 mass% or less, REM: 0.010 mass% or less, W: 0.40 mass% or less, Co: The marine or marine vessel according to claim 1, comprising one or more selected from 0.50 mass% or less, Mg: 0.005 mass% or less, and Zr: 0.02 mass% or less. Thick steel plate for structures and hydraulic iron pipes. The average value of equivalent circle diameters of crystal grains surrounded by a large-angle grain boundary with a misorientation angle of 15 degrees or more at a plate thickness 1/4 position is 5 μm or less, and the average grain size of prior austenite grains is 30 μm or less. The thick steel plate for ships, offshore structures and hydraulic iron pipes according to claim 1 or 2 , wherein the variation σ is 15 µm or less. A slab comprising the component composition according to claim 1 or 2 is hot-rolled, and then heated at a thickness of 1/4 t in a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point during reheating. Is heated at 1 to 30 ° C./min, reheated to a temperature of Ac ₃ transformation point to (Ac ₃ transformation point + 70 ° C.), and then subjected to quenching treatment at least once, and then a temperature of 550 to 700 ° C. in and facilities tempering process, the microstructure in the sheet thickness 1/4 position consists tempered lower bainite and tempered martensite structure, and a tensile strength of more than 570 MPa, 720 MPa or less, NRL in the sheet thickness 1/4 position marine, offshore structures and for penstock for steel plate manufacturing method of T _NDT of drop weight test, characterized in that to obtain the following steel plate -55 ° C.. 5. The method for producing thick steel plates for marine vessels, marine structures, and hydraulic iron pipes according to claim 4 , wherein the tensile strength as-quenched at a ¼ thickness position is 850 MPa to 1100 MPa.

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