JP4283757B2 - Thick steel plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength thick steel plate of 590 to 780 MPa class excellent in strength-ductility balance and weldability and a method for producing the same.

  Conventionally, large structures such as ships, offshore structures, bridges, and building structures have been lightened, and high-strength steel sheets of 590 MPa class or higher have been used for these structural and structural steel sheets. It has been demanded. Such high-strength thick steel plates are also required to have high uniform elongation, particularly from the viewpoint of improving the earthquake resistance of building structures and steel structures. This uniform elongation means the elongation until the local contraction starts in the middle of the steel sheet until it breaks, and is an index of stability when the steel sheet is deformed, and the value of the uniform elongation is high. It is said that better earthquake resistance is obtained.

  As means for improving the uniform elongation, it has been conventionally known to increase the amount of retained austenite (residual γ) by utilizing transformation-induced plasticity (hereinafter referred to as “TRIP”) of austenite.

  For example, in the field of hot-rolled high-tensile steel sheet as a material for high-strength members used in automobiles and various industrial machines, it is processed into a predetermined shape by forming such as pressing, so it has an excellent strength-ductility balance. Steel plate is required. For this reason, for example, Patent Documents 1 to 3 disclose steel manufacturing methods that combine a suitable structure refinement with the TRIP phenomenon of retained austenite and have an excellent strength-ductility balance.

  Moreover, in patent document 4, about the hot-rolling high-tensile steel plate which contains 0.05 to 0.30% of C as a hot-rolled high-tensile steel plate which is excellent in strength-ductility balance and can be subjected to chemical conversion treatment, by volume ratio. It has been proposed that 15% or more of austenite is contained, and the balance is substantially composed of polygonal ferrite having an average crystal grain size of 1.5 to 3 μm.

  These patent documents 1 to 4 are steel plates used for automobiles and various industrial machines, unlike thick steel plates that are subjected to high heat input welding, and naturally do not consider the HAZ toughness during high heat input welding. For this reason, naturally, since the HAZ toughness at the time of high heat input welding is low, it cannot be applied to a thick steel plate for large structures such as buildings and bridges. Also, as a structure, polygonal ferrite lowers the strength-ductility balance of a high-strength thick steel plate, weld crack resistance, and high heat input HAZ toughness.

  Further, when the residual γ is increased in order to improve the uniform elongation, the island-like martensite also increases and the base material toughness decreases.

Therefore, as a technique for improving uniform elongation while ensuring good base metal toughness even in a thick steel plate welded with high heat input, in Patent Document 5, C: 0.010 to 0.06% of 590 MPa class In the high-tensile steel plate, 0.5% by volume or more of retained austenite is present, and the island-like martensite fraction is 20% by volume or less, expressed by [Mn] + 1.5 × [Cr] + 2 × [Mo]. It has been proposed to set the KP value (%) to be in a specific range.
JP 63-4017 A (Claims) JP-A-9-87798 (Claims) JP-A-9-104947 (Claims) JP 2004-131833 A (Claims) Japanese Patent Laying-Open No. 2003-160835 (Claims, Table 3)

  However, according to the above-mentioned Patent Document 5, the tensile strength is 618 MPa and the uniform elongation is about 13.1% as shown in Table 3. The ductility is low for the tensile strength, and the strength-ductility balance (tensile strength × uniform elongation). ) Is only at a low level.

  For these reasons, in a high strength thick steel plate of 590 MPa class or higher, a thick steel plate with improved uniform elongation and excellent strength-ductility balance is desired while ensuring good base metal toughness and high heat input weldability. The reality is.

  The present invention has been made in view of such circumstances, and its purpose is to improve the uniform elongation while ensuring good base material toughness and large heat input weldability, and excellent strength-ductility balance. Another object of the present invention is to provide a high strength thick steel plate of 590 to 780 MPa class and a method for producing the same.

To this end, the gist of the steel plate of the present invention, in mass%, C: 0.01~0.10%, Si : 0.05~2.0%, Mn: 1.5~7 0.0%, Al: 0.1% or less (excluding 0%), Ti: 0.002 to 0.1%, N: 0.001 to 0.01%, respectively, the balance being iron and Thick steel plate which is an inevitable impurity, the fraction of residual γ in the thick steel plate structure is 1.0 to 30% by volume, the fraction of polygonal ferrite is 0 to 26% by volume, and the balance is bainite And the fraction of residual γ satisfies the following KTP value.
Here, KTP value = −3.14 × 10 ³ + 163 × [γR fraction] + 5.09 × 10 ⁵ × (1 / Ms [γR]), and 0 ≦ KTP value ≦ 2730.
However, Ms [γR] is the Ms point of residual γ (martensitic transformation start temperature),
Ms [γR] = 550-361 [% C (γR)]-39 [% Mn] -20 [% Cr] -17 [% Ni] -10 [% Cu] -5 [% Mo]
However,% C (γR) is the amount of C in the residual γ.

In the Ms [γR] formula, the present invention includes and takes into account the amounts of Cr , Ni, Cu, and Mo. This, Cr, C u, not only the case where Mo selectively include substantial amounts, Cr, Ni, Cu, also include a measurable impurity levels Mo, strictly to the value of Ms [γR] Because it has an influence, it comes from the recognition that it should be taken into account in the calculation of important Ms [γR].

Moreover, in order to achieve this objective, the summary of the manufacturing method of the thick steel plate excellent in the strength-ductility balance and weldability of this invention is the steel raw material which consists of one of the component compositions of Claims 1-6. Then, after heating and hot rolling, forced cooling is performed, and then, heat treatment is performed by heating and holding at a heating temperature T _{hold in} which (T _hold −Ae 1) / (Ae 3 −Ae 1) × 100 is in the range of 5-50. That is.

  In the present invention, in the high-strength 590-780 MPa class thick steel plate, in order to improve uniform elongation, the amount of residual γ is increased or secured in common with the prior art.

  However, in the present invention, the amount of C in the residual γ is further increased (the C concentration is increased) to ensure the stability of the residual γ. This makes it possible to balance the fraction (amount) of residual γ and the stability of residual γ, improve uniform elongation, and secure an excellent strength-ductility balance. Moreover, the increase of island-like martensite can also be suppressed and toughness can be ensured.

  Therefore, the KTP value in the present invention can be said to be an indicator of the stability of residual γ, an indicator of the transformation induced plasticity (“TRIP”) effect of austenite, and an indicator of the strength-ductility balance.

  The strength-ductility balance (tensile strength x uniform elongation) of Patent Document 5 described above is low because the fraction (amount) of residual γ is secured, but the amount of C in the residual γ is small. Is considered to be unstable.

  Thus, the 590-780 MPa class high-strength thick steel plate of the present invention improves the uniform elongation while ensuring good base metal toughness and high heat input weldability, and has excellent strength-ductility balance, and good earthquake resistance. Sex is obtained. As a result, it is optimal for use in large structures such as ships, offshore structures, bridges, and building structures.

First, in the 590-780 MPa class high-strength thick steel plate of the present invention, the structure that becomes the main phase is bainite. The bainite structure ensures good base material toughness and high heat input weldability, and ensures uniform elongation improvement, good strength-ductility balance, good earthquake resistance, and the like. For this reason, the thick steel plate has a bainite-based structure including residual γ .

  However, due to manufacturing costs and limitations, the generation or mixing of polygonal ferrite, martensite, or cementite is permitted within the range that does not impair the above characteristics in addition to the bainite. However, these structures are preferably as small as possible because they reduce the strength-ductility balance. In particular, polygonal ferrite, which is likely to be generated or mixed in the cooling process after rolling, has a steel sheet microstructure fraction of 15% in terms of strength-ductility balance, weld crack resistance and high heat input HAZ toughness. The following is preferable.

  As for the fraction (volume fraction) of the transformation structure such as polygonal ferrite and bainite, the surface thickness of each steel sheet ¼ part was ground and then corroded with 3% nital corrosive solution, and then the structure was examined by an optical microscope. (Magnification: 1000 times), an area of 50 μ square is photographed at n = 10, and measured by image analysis such as a point counting method.

(Residual γ)
Next, the regulation of residual γ in the steel plate structure in the present invention will be described below. The fraction of residual γ in the steel sheet structure of the present invention is 1.0 to 30% in terms of volume fraction on the premise of obtaining an excellent strength-ductility balance. If the fraction of residual γ is less than 1.0%, the “TRIP” effect of residual γ is not exhibited. As a result, as a premise, in a high strength thick steel plate of 590 to 780 MPa class, an excellent strength-ductility balance with a tensile strength × uniform elongation of 14,000 or more cannot be obtained. On the other hand, when the fraction of residual γ exceeds 30%, island martensite tends to increase and toughness decreases.

(KTP value)
In the present invention, as described above, the amount of C in the residual γ is increased (the C concentration is increased) to ensure the stability of the residual γ. Thereby, the fraction (amount) of residual γ and the stability of residual γ can be balanced, and uniform elongation can be improved. As a result, in a high strength thick steel plate of 590 to 780 MPa class, an excellent strength-ductility balance with a tensile strength × uniform elongation of 14,000 or more can be ensured.

The Ms point (martensite transformation start temperature) of the residual γ indicates the stability of the residual γ, and the higher the amount of C in the residual γ, the more each amount of Mn, Cr, Ni, Cu, etc. as follows Ms [gamma _R] expression is Ms point of the residual gamma Ms [gamma _R] is reduced, the residual gamma is stabilized.

As shown in the following KTP value equation, the lower the Ms [γ _R ] and the higher the γ _R fraction, the higher the KTP value that is the sum of the reciprocal of this Ms [γ _R ] and the γ _R fraction is Greater than zero. Therefore, the following KTP value can be said to be an index of stability of residual γ, an index of austenite transformation-induced plasticity (“TRIP”) effect, and an index of strength-ductility balance. The fraction (amount) of residual γ can be balanced with the stability of residual γ.

On the other hand, if the KTP value is less than 0, the C amount level in the residual γ decreases, Ms [γ _R ], which is the Ms point of the residual γ, increases, and the residual γ becomes unstable. For this reason, strength-ductility balance falls.
KTP value = −3.14 × 10 ³ + 163 × [γ _R fraction] + 5.09 × 10 ⁵ × (1 / Ms [γ _R ]) ≧ 0
Where Ms [γ _R ] is the Ms point of residual γ (martensitic transformation start temperature),
Ms [γ _R ] = 550-361 [% C (γ _R )]-39 [% Mn] -20 [% Cr] -17 [% Ni] -10 [% Cu] -5 [% Mo] The
However,% C (γ _R ) is the amount of C in the residual γ.

This equation for the KTP value itself is used to quantitatively evaluate the effect of the amount of residual γ (γ _R ) and the stability of residual γ on the decrease in “TRIP”, so the amount of residual γ and the stability of residual γ Various samples with different properties (replaced by the Ms point of residual γ) were made, and a multiple regression equation was created from the TS (tensile strength) × EL (uniform elongation) obtained from these samples and the above parameters, This is a formula normalized so that the condition of TS × EL = 14000 (critical condition of strength-ductility balance) becomes zero.

(Measurement of residual γ)
The fraction of the residual γ can be measured from the X-ray diffraction measurement of the steel sheet structure at a ¼ thickness portion of the steel sheet. That is, for example, using an X-ray diffraction measuring device (RAD-RU300 manufactured by Rigaku Corporation), setting the target as Co and the target output as 40 kV-200 mA, obtain the X-ray diffraction peak of the steel sheet structure, and calculate the theoretical intensity ratio by the Liberty method. The residual γ amount (Vγ amount) is determined by calculation.

Further,% C (γ _R ), which is the amount of C (C concentration) in the residual γ, is obtained by applying Si as a standard material to a steel sheet sample and measuring the X-ray diffraction to obtain Si, residual γ (γ _R ) X-ray diffraction peak position is determined. Using this peak position, the lattice constant a ₀ of the residual γ is measured. The usage peaks are (111), (200), (220), and (311).

From the lattice constant a ₀ , the amount of C (C concentration) in the residual γ is calculated by the following equation described in “DJDyson et al., Journal of The Iron and Steel Institute, (1970) p469-474”. Ask. The following equation is calculated with respect to the amount of C in the residual γ, with other carbide forming elements contained in the steel sheet as a negative factor.
% C (γ _R ) = (a ₀ −3.578−0.00095 ×% Mn + 0.0002 ×% Ni−0.0006 ×% Cr−0.022 ×% N−0.0056 ×% Al-0. 0004 ×% Co−0.0015 ×% Cu−0.0031 ×% Mo−0.0051 ×% Nb−0.0039 ×% Ti−0.0018 ×% V−0.0018 ×% W) / 0. 033

(Composition of thick steel plate)
The composition (unit:% by mass) of the thick steel plate of the present invention will be described below including the reasons for limiting each element. As a premise of controlling the above-mentioned structure of the steel plate of the present invention and improving the uniform elongation while ensuring good base material toughness and large heat input weldability, excellent strength-ductility balance, and obtaining good earthquake resistance, The composition of the steel plate of the present invention is within the range shown below, and it is effective to manufacture it by a prescribed method.

That is, in mass%, C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 1.5 to 7.0%, Al: 0.1% or less (0% Not included), Ti: 0.002 to 0.1%, N: 0.001 to 0.01%, respectively, with the balance being iron and inevitable impurities.

Hereinafter, the reason for defining the amount of each element will be described in detail.
C: 0.01 to 0.10%.
C (carbon) exhibits the “TRIP” effect, increases the amount of C in the residual γ (increases the C concentration), ensures the stability of the residual γ, and sets the KTP value to 0 or more. In a high strength thick steel plate of ˜780 MPa class, it is an important element for ensuring an excellent strength-ductility balance of tensile strength × uniform elongation of 14,000 or more. Furthermore, C is also effective for ensuring the weld crack resistance of the HAZ part during welding, high heat input HAZ toughness, and ensuring the strength of the base material.

In order to exert such an effect, at least 0.01% is necessary. If the C content is less than 0.01%, the “TRIP” effect is not exhibited, and the amount of C in the residual γ decreases, and the residual γ Ms [γ _R ], which is the Ms point, increases, the residual γ becomes unstable, and the KTP value becomes less than 0. For this reason, in a high-strength thick steel plate of 590 to 780 MPa class, it becomes impossible to ensure an excellent strength-ductility balance of tensile strength × uniform elongation of 14,000 or more.

  On the other hand, when the amount of C exceeds 0.10% and becomes excessive, martensite is generated instead of low-temperature transformation bainite on the high cooling rate side. On the other hand, weld crack resistance and high heat input HAZ toughness are not improved. . Therefore, the C content is 0.01 to 0.10%, preferably 0.02 to 0.08%.

Si: 0.05-2.0%.
Si has an effect of suppressing the formation of cementite and improves the strength-ductility balance. In addition, solid solution strengthening contributes to securing the strength of the base material. This effect is exhibited when the content is 0.05% or more, preferably 0.2% or more. On the other hand, if it exceeds 2.0% and is contained excessively, both the base metal toughness and the HAZ toughness are lowered. For this reason, Si content is 0.05 to 2.0%, preferably 0.2 to 2.0%.

Mn: 1.5 to 7.0%.
Mn has the effect of improving the hardenability of the steel and makes it easy to generate low temperature transformation bainite at a high cooling rate or a low cooling rate. When the Mn content is less than 1.0%, the desired hardenability improving effect is not exhibited, the residual γ is destabilized, and the strength of the base material is insufficient, so that the strength-ductility balance is lowered. On the other hand, if it is contained excessively over 7.0%, the HAZ toughness deteriorates. Therefore, the range of the Mn content is in the range of 1.5 to 7.0%, preferably in the range of 2.0 to 6.0%.

Al: 0.1% or less (excluding 0%).
Al fixes the solid solution nitrogen as AlN and enhances the strength-ductility balance by solid solution strengthening. On the other hand, when Al is contained excessively exceeding 0.1%, the solid solution strengthens too much and the base material properties such as toughness are deteriorated. For this reason, the Al content is determined by the amount of nitrogen, and when there is no nitrogen, it is not necessary to contain it in particular. Therefore, the Al content is 0.1% or less (not including 0%), preferably 0.05% or less (not including 0%).

Ti (total amount): 0.002 to 0.1%.
Ti is useful in that Ti forms nitrogen and nitride, or oxygen and oxide, refines the γ grains in the HAZ part during high heat input welding, and contributes to the improvement of HAZ toughness. In order to exhibit such an effect effectively, Ti (total amount) is contained by 0.002% or more. On the other hand, if the amount of Ti is Ti (total amount) exceeding 0.1%, Ti nitride and Ti oxide become excessive or coarse, and on the contrary, both HAZ toughness and base metal toughness deteriorate. To do. Therefore, the total Ti content (total amount) is 0.002 to 0.1%, preferably 0.005 to 0.05%.

N: 0.001 to 0.01%.
N (nitrogen) is useful in that Ti forms nitrogen and nitride, or oxygen and oxide, refines the γ grains in the HAZ part during high heat input welding, and contributes to the improvement of HAZ toughness. In order to exhibit such an effect effectively, 0.001% or more is contained. On the other hand, if the N amount exceeds 0.01%, the base material toughness and the HAZ toughness are both deteriorated. Therefore, the N content is in the range of 0.001 to 0.01%, preferably 0.0030 to 0.0080%.

Hereinafter, elements to be selectively contained will be described.
Any one or more of Cr , Cu and Mo.
Cr , Cu, and Mo all become negative terms in the expression of Ms [γR] in the KTP value, stabilize residual γ, and increase the strength-ductility balance. In order to exert this effect, one or more of Cr , Cu, and Mo are selectively contained in a total amount of 0.2% or more.

On the other hand, if one or more of these elements exceeds 5% in total and excessively contained, the residual γ becomes too stable and the “TRIP” effect cannot be obtained. Therefore, when one or more of Cr , Cu and Mo are selectively contained, the total content is 0.2 to 5%, preferably the total content is 0.5 to 3.0%. .

B: 0.0005 to 0.0050%.
B suppresses the formation of polygonal ferrite in the cooling process after rolling, and ensures the strength of the base material. In addition, there is an effect of improving the base material toughness by refining the steel structure. This effect is exhibited when the content is 0.0005% or more. On the other hand, when the B content exceeds 0.0050%, these effects are saturated. Therefore, the B content is in the range of 0.0005 to 0.0050%.

Any one or more of Nb, V, Zr and W.
Nb, V, Zr, and W have the effect of forming carbide (MC) and increasing the strength of the base material. In order to exert this effect, one or more of Nb, V, Zr and W are selectively contained in a total of 0.01% or more.

  On the other hand, if one or more of these elements exceeds 0.5% in total and excessively contained, MC increases too much, free carbon in steel decreases, and stability of residual γ Reduce. Therefore, in the case where one or more of Nb, V, Zr, and W are selectively contained, the total content is made 0.01 to 0.5%.

REM: 0.001 to 0.1%.
REM refines the inclusions such as sulfides such as MnS to improve the HAZ toughness. In order to exhibit this effect, 0.001% or more is selectively contained. However, even if REM exceeds 0.1%, the effect is saturated. Therefore, when REM is contained selectively, the content is made 0.001 to 0.1%.

  Next, inevitable impurities will be described below. Elements other than the above are impurities, and inclusion in a range that does not impair the properties of the thick steel plate is allowed. For example, P (phosphorus) and S (sulfur) are elements present as unavoidable impurities, and have an adverse effect such as reducing weldability and base metal toughness. Therefore, it is preferable to keep P at 0.020% or less and S at 0.010% or less.

(Production method)
The steel plate of the present invention can be produced by a conventional method including the hot rolling. That is, the steel material is melted by a normal melting method such as a converter and then a steel material (slab) having a predetermined size is formed by a normal casting method such as a continuous casting method. The steel material (slab) is a structure that is recrystallized at the end of the hot rolling by performing hot rolling after heating, preventing the development of the texture along the rolling direction, in accordance with the normal method for producing thick steel plates. To do. The steel sheet after hot rolling is subjected to water quenching. Thereafter, the steel plate is tempered to obtain a product thick steel plate.

  Although rolling conditions are not specifically limited, Preferably, after heating to 1000-1200 degreeC, a finish rolling temperature is made 700-900 degreeC and rolled. By such low temperature rolling, the structure after heat treatment in the two-phase region described later can be refined, and characteristics such as base material toughness can be improved.

  The cooling rate in forced cooling such as water quenching after rolling is preferably 1.0 to 20 ° C./s. Thus, by increasing the cooling rate and increasing the fraction of bainite, the structure after heat treatment in the two-phase region described later can be refined, and characteristics such as base material toughness can be improved.

In order to satisfy the conditions of the structure, attention is particularly paid to the temperature conditions during the heat treatment (tempering, tempering). The temperature of the heat treatment is overheated to the α + γ2 phase region between Ae1 and Ae3. Specifically, the heat treatment temperature (heating temperature: T _hold : ° C.) is heated so that (T _hold −Ae 1) / (Ae 3 −Ae 1) × 100 (%) is in the range of 5 to 50%. Hold. By holding at this temperature, fine austenite can be made, and then, when cooled to room temperature, a bainite structure in which a residual γ is formed in a volume fraction of 1.0 to 30% can be obtained. The holding time is set to a time sufficient to obtain these effects, but is preferably held for 3 minutes or more.

Heating temperature: When T _hold is (T _hold −Ae1) / (Ae3−Ae1) × 100 and less than 5%, α + θ region less than Ae1, A sufficient amount of residual γ of 1.0% or more cannot be secured.

On the other hand, when the temperature of the heat treatment exceeds 50% at (T _hold −Ae1) / (Ae3−Ae1) × 100, exceeds Ae3 and becomes the γ region, the residual γ is reduced in volume when cooled to room temperature. Since the fraction exceeds 30% and carbon cannot be sufficiently concentrated in the residual γ, the stability of the residual γ is lowered, and on the contrary, a sufficient amount of the residual γ cannot be secured.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention is not limited to the following examples. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having chemical composition shown in Table 1 (Invention Examples A to P and Comparative Examples Q to W) were vacuum melted to prepare 150 kg ingots. This ingot was subjected to multi-pass rolling and forced cooling under the rolling conditions shown in Table 2 to obtain a steel plate having a thickness of 30 mm. This steel plate was heat-treated under the two-phase region heat treatment conditions shown in Table 2 and Table 3 (heating time is commonly about 1 hour) to obtain a test material. Table 1 shows the values calculated by thermocalc as thermodynamic software for Ae1 and Ae3 of each steel plate.

The heating conditions for the two-phase region heat treatment in Table 2 include the conditional expression of the heating temperature (T _hold : ° C.) in the two-phase region heat treatment shown in Table 3, (T _hold −Ae1) / (Ae3−Ae1). ) × 100 (%) is described together with an error range (±%) based on the error of the heating temperature.

A sample was taken from the steel plate thus obtained, and as shown in Table 3, the volume fraction of polygonal ferrite (α fraction:%) and the fraction of residual γ (γ _R fraction) in the thick steel plate structure. :%), The amount of C in the residual γ [C (γ _R ):%], the Ms point of the residual γ (Ms [γ _R ]: ° C.), and the KTP value were determined by the measurement method and calculation method described above, respectively. . It was also confirmed whether the remaining structure was mainly bainite.

  And the base material tensile characteristics and weldability of the same sample were measured. These results are shown in Table 3.

(Base material tensile properties)
A JIS4A test piece was taken from the above sample and subjected to a tensile test according to JISZ2241, and the tensile strength (TS: MPa) of the steel sheet and uniform elongation (EL: strain amount when load decreased by 5% from the maximum value uE Further, the strength-ductility balance of TS × EL was also obtained, and it was evaluated that the strength-ductility balance (MPa%) of 14000 or more was excellent.

(Base material toughness)
A Charpy test piece was cut out from the depth portion of the plate thickness t / 4, a Charpy impact test according to JISZ2242 was performed, and the toughness (vE0: J) at 0 ° C was measured. The case where vE0 was 110 J or more was evaluated as being excellent in base metal toughness.

(Welded joint toughness)
A heat cycle in which a test piece (size: 12.5 mm × 32 mm × 55 mm) cut out from the sample is heated to 1400 ° C. and 1200 ° C., held at the temperature for 5 seconds, and then cooled from 800 ° C. to 500 ° C. for 730 seconds. (Corresponding to the thermal history of HAZ when SAW welding is performed with a heat input of 5 kJ / mm). Charpy test pieces were collected from each of these test pieces and subjected to a Charpy impact test in accordance with JISZ2242, and the toughness (vE0: J) at 0 ° C. was measured. The case where vE0 was 100 J or more was evaluated as being excellent in weld joint toughness.

  As is apparent from Tables 1 to 3, Invention Examples 1 to 17 use the steels of Invention Examples A to P in Table 1 that satisfy the composition of the present invention, and manufacture of preferable two-phase region heat treatments 1 and 2 in Table 2. Manufactured in a range of conditions. For this reason, the fraction of residual γ in the steel plate structure is in the range of 1.0 to 30%, and the fraction of residual γ satisfies the following KTP value. In addition, the steel sheet structures of Invention Examples 1 to 17 are bainite, with the remainder excluding the γR fraction (residual γ fraction) and α fraction (polygonal ferrite fraction) shown in Table 3 being mainly bainite. It was an organization.

  As a result, in a high-strength thick steel plate of 590 MPa class or higher, a strength-ductility balance of 14000 MPa% or higher is obtained. Moreover, it is excellent in base material toughness. Furthermore, a thermal cycle characteristic of toughness of 100 J or more is obtained, and weldability such as weld joint toughness is also excellent.

  These results show that good earthquake resistance is obtained when used as a high strength 590-780 MPa class thick steel plate in a building structure or a steel structure.

  On the other hand, Comparative Examples 18 to 24 use steels of Comparative Examples Q to W in Table 1 in which the component composition of any element falls outside the scope of the invention. For this reason, even if the production conditions such as rolling and two-phase region heat treatment are produced within a preferable range, even if the structure regulation such as the fraction of residual γ is out of or included, the strength of 14000 MPa% or more− Either or all of the properties of ductility balance, base material toughness, and thermal cycle properties are inferior to those of the inventive examples.

  Further, Comparative Example 25 is manufactured outside the preferable range of manufacturing conditions such as rolling, two-phase region heat treatment, etc., despite using the steel of Invention Example A in Table 1 that satisfies the composition of the present invention, The structural regulation such as the fraction of residual γ is not satisfied, and either the strength-ductility balance of 14000 MPa% or more, the thermal cycle characteristics, or both characteristics are inferior to the invention examples.

  As a result, these comparative examples cannot be used as 590-780 MPa class thick steel plates for building structures and steel structures that require earthquake resistance.

  In Comparative Example 18, the C amount of steel Q is too high and deviates from the upper limit. For this reason, although the structure such as the fraction of residual γ is within the scope of the invention, the thermal cycle characteristics are low and the weldability is poor.

  In Comparative Example 19, the amount of Si in steel R is too high and deviates from the upper limit. For this reason, although the structure such as the fraction of residual γ is within the scope of the invention, the thermal cycle characteristics are low and the weldability is poor.

  In Comparative Example 20, the amount of Si in steel S is too low and deviates from the lower limit. For this reason, the residual γ is small, the fraction of the residual γ does not satisfy the KTP value, and the strength-ductility balance is inferior.

  In Comparative Example 21, the amount of Mn in the steel T is too low and falls below the lower limit. For this reason, the fraction of residual γ does not satisfy the KTP value, and the strength-ductility balance is inferior.

  In Comparative Example 22, the Mn amount of the steel U is too high and deviates from the upper limit. For this reason, although the structure such as the fraction of residual γ is within the scope of the invention, the thermal cycle characteristics are low and the weldability is poor.

  In Comparative Example 23, the Al amount of the steel V is too high and deviates from the upper limit. For this reason, the amount (α fraction) of polygonal ferrite increases, the fraction of residual γ does not satisfy the KTP value, and the strength-ductility balance and thermal cycle characteristics are inferior.

  In Comparative Example 24, the Ti amount of the steel W is too high and deviates from the upper limit. For this reason, Ti carbides are generated, the residual γ is not formed, and the strength-ductility balance and thermal cycle characteristics are inferior.

  Comparative Example 25 is manufactured by a two-phase heat treatment outside the preferable range of 3 in Table 2, although the steel of Invention Example A in Table 1 that satisfies the composition of the present invention is used. For this reason, since the fraction of residual γ is small, the KTP value is not satisfied, and the strength-ductility balance and thermal cycle characteristics are inferior.

  The above results support the critical significance of strength-ductility balance and weldability toughness improvement in the case of high-strength 590-780 MPa grade thick steel sheets with the component composition and structure defined in the present invention.

As described above, according to the present invention, a high strength thick steel plate of 590 to 780 MPa class that improves uniform elongation and has excellent strength-ductility balance while ensuring good base material toughness and large heat input weldability. And a manufacturing method thereof. For this reason, this invention steel plate is applicable to the structure and building structure with which earthquake resistance is requested | required.

Claims (6)

In mass%, C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 1.5 to 7.0%, Al: 0.1% or less (excluding 0%) ), Ti: 0.002 to 0.1%, N: 0.001 to 0.01%, respectively, and the balance is a steel plate with iron and inevitable impurities, and the residual γ in the steel plate structure The fraction is 1.0 to 30% by volume, the fraction of polygonal ferrite is 0 to 26% by volume, is composed of the remainder bainite, and the fraction of residual γ satisfies the following KTP value. steel plate you, characterized in that.
Here, KTP value = −3.14 × 10 ³ + 163 × [γR fraction] + 5.09 × 10 ⁵ × (1 / Ms [γR]), and 0 ≦ KTP value ≦ 2730.
However, Ms [γR] is the Ms point of residual γ (martensitic transformation start temperature),
Ms [γR] = 550-361 [% C (γR)]-39 [% Mn] -20 [% Cr] -17 [% Ni] -10 [% Cu] -5 [% Mo]
However,% C (γR) is the amount of C in the residual γ. Steel plate of claim 1 wherein the thick steel further, the Cr, C u, more either one or two of Mo, containing from 0.2 to 5% in total. The steel plate further, B: steel plate according to claim 1 or 2 containing 0.0005 to 0.0050%. The steel plate further, Nb, V, Zr, and any one or more one or two of W, the thickness of any one of claims 1 to 3 containing 0.01-0.5% in total steel sheet. The steel plate is further steel plate according to any one of claims 1 to 4 containing REM 0.001 to 0.1%. A steel material having the composition of any one of claims 1 to 5 is heated and forcibly cooled after hot rolling, and thereafter (Thold-Ae1) / (Ae3-Ae1) × 100 is 5 to 50. method for producing a steel plate you and performing heat treatment for holding and heating the heating temperature Thold as a range.