KR20060052433A - Thick steel plate excellent in strength-ductility balance and welding property, and process for producing the same - Google Patents

Abstract

An object of the present invention is to provide a high-strength thick steel sheet of 590 to 780 MPa grade having excellent strength and ductility balance, and improving its uniform elongation while securing good base material toughness and high heat input weldability, and a method of manufacturing the same. The thick steel sheet having an improved strength-ductility balance according to the present invention is C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 1.5 to 7.0%, Al: 0.1% or less (does not include 0%), Ti : 0.002 to 0.1%, N: 0.001 to 0.01%, the remainder of which is substantially iron and unavoidable impurities, after hot rolling, (T _hold -Ae1) / (Ae3-Ae1) x A heat treatment was carried out to heat and _hold at a heating temperature T _hold in the range of 5 to 50 at 100, and the fraction of residual γ in the thick steel sheet structure was 1.0 to 30%. It is made to satisfy.

Description

THICK STEEL PLATE EXCELLENT IN STRENGTH-DUCTILITY BALANCE AND WELDING PROPERTY, AND PROCESS FOR PRODUCING THE SAME}

The present invention relates to a high-strength thick steel sheet of 590 to 780 MPa grade, which is excellent in strength-ductility balance and weldability, and a manufacturing method thereof.

Conventionally, weight reduction is aimed at large structures, such as a ship, an offshore structure, a bridge, and a building structure, and the high strength thick steel plate of 590 MPa grade or more is requested | required by the thick steel plate for these structures and structures. Such a high strength thick steel sheet is also required to have high uniform elongation, particularly from the viewpoint of improving the seismic resistance of a building structure or a steel structure. This uniform elongation means the elongation until local shrinkage starts in the middle of the steel sheet breaking, and is an index of stability when the steel sheet deforms. The higher the value of the uniform elongation, the better the seismic resistance. It is said to be obtained.

As a means of improving this uniform elongation, it has conventionally been known to increase the amount of retained austenite (residual γ) by using transformed organic calcination of austenite (hereinafter referred to as "TRIP").

For example, in the field of hot-rolled high tensile strength steel sheet as a raw material for high strength members used in automobiles and various industrial machines, a steel sheet having excellent strength-ductility balance is required to be processed into a predetermined shape by molding processing such as press working. For this reason, the manufacturing method of the steel which is excellent in strength-ductility balance which combines refinement | miniaturization of a preferable structure and TRIP phenomenon of residual austenite, for example, Unexamined-Japanese-Patent No. 1988-4017 (claim) and Japanese patent Japanese Patent Laid-Open No. 1997-87798 (claims) and Japanese Patent Laid-Open No. 1997-104947 (claims).

In addition, Japanese Patent Application Laid-Open No. 2004-131833 (claims) discloses a hot rolled high tensile strength steel sheet containing 0.05 to 0.30% of C as a hot rolled high tensile steel sheet which is excellent in strength-ductility balance and can be subjected to chemical conversion. On the other hand, it is proposed to contain 15% or more of austenite in a volume ratio, and the balance consists of polygonal ferrite having an average crystal grain size of 1.5 to 3 m.

These Japanese Patent Publication Nos. 1988-4017, 1997-87798, 1997-104947 and 2004-131833 are steel sheets used in automobiles and various industrial machines, unlike high-heat welded thick steel sheets. Of course, HAZ toughness is not considered. For this reason, of course, HAZ toughness at the time of high heat input welding is low, and it cannot apply to thick steel plates for large structures, such as a building and a bridge. Also, as a structure, polygonal ferrite reduces the strength-ductility balance, weld cracking resistance, and high heat input HAZ toughness of the high strength thick steel sheet.

Moreover, when residual (gamma) is increased in order to improve uniform elongation, island-like martensite also increases and it becomes a problem that base material toughness falls.

On the other hand, even in a high-heat welded thick steel sheet, as a technique for improving uniform elongation while ensuring good base material toughness, C: 0.010 to 0.06% in Japanese Patent Laid-Open No. 2003-160835 (claim 3, Table 3). In the 590 MPa class high tensile steel sheet of the present invention, at least 0.5% by volume of retained austenite is present, and the martensite fraction on the island is set to 20% by volume or less, and [Mn] + 1.5 × [Cr] + 2 × [Mo]. It is proposed to make KP value (%) shown by the specific range.

However, Japanese Patent Laid-Open No. 2003-160835 discloses a uniform elongation of about 13.1% at a tensile strength of 618 MPa as shown in Table 3 of the present invention. Uniform elongation) should only be low.

Therefore, in the high strength thick steel plate of 590 MPa grade or more, it is a fact that the thick steel plate excellent in intensity-ductility balance is desired, improving uniform elongation, ensuring favorable base material toughness and high heat input weldability.

This invention is made | formed in view of such a situation, The objective is the high-strength thick steel plate of 590-780 MPa grade excellent in intensity-ductility balance, improving uniform elongation, ensuring good base material toughness and high heat input weldability, and its It is to provide a manufacturing method.

In order to achieve this object, the gist of the thick steel sheet excellent in strength-ductility balance and weldability of the present invention is, in mass%, C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 15 to 7.0%, Al : 0.1% or less (does not contain 0%), Ti: 0.002 to 0.1%, N: 0.001 to 0.01%, respectively, the remainder being iron and unavoidable impurities, respectively. The fraction of residual gamma is 1.0 to 30%, and the fraction of this residual gamma satisfies the following KTP value.

KTP value = -3.14 x 10 ³ + 163 x [γ _R fraction] + 5.09 x 10 ⁵ x (1 / Ms [γ _R ]) ≥0.

However, Ms [γ _R ] is the Ms point (martensite transformation start temperature) of residual γ,

Ms [γ _R ] = 550-361 [% C (γ _R )]-39 [% Mn]-20 [% Cr]-17 [% Ni]-10 [% Cu]-5 [% Mo] .

However,% C (γ _R ) is the amount of C in the residual γ.

In the formula of Ms [γ _R ], on the other hand, in the present invention, the amounts of Cr, Ni, Cu, and Mo, which are optional addition elements, are included and considered. Not only does this selectively contain (add) the actual mass of Cr, Ni, Cu, and Mo, but also contains it as a measurable impurity level, so that the value of Ms [γ _R ] is strictly affected. This is from the perception that this should be considered in the calculation of important Ms [γ R].

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 heats and hot-rolls the steel material which consists of a component composition of any one of Claims 1-6. After that, forced cooling is performed, and then, heat treatment is performed to heat and _hold at a heating temperature T _hold in which (T _hold -A _el ) / (A _e3 -A _e1 ) × 100 is in a range of 5 to 50.

In the present invention, in the high strength 590 to 780 MPa class thick steel sheet, it is common to increase or secure the amount of residual γ in order to improve uniform elongation.

However, in the present invention, the amount of C in the residual γ is also increased (by increasing the C concentration) to ensure the stability of the residual γ. Thereby, the balance of the fraction (amount) of residual (gamma) and the stability of residual (gamma) can be balanced, a uniform elongation can be improved and the outstanding strength-ductility balance can be ensured. In addition, the increase in the island-like martensite can also be suppressed to ensure toughness.

Therefore, it can be said that the said KTP value in this invention is an index of stability of residual (gamma), the index of the transformation organic plasticity ("TRIP") effect of austenite, or the index of a strength-ductility balance.

The low strength-ductility balance (tensile strength x uniform elongation) of Japanese Patent Laid-Open No. 2003-160835 described above secures a fraction (amount) of residual γ, but because the amount of C in residual γ is small, the residual γ becomes unstable. I think it is because there is.

The high strength thick steel sheet of 590 to 780 MPa class of the present invention improves uniform elongation while securing good base material toughness and high heat input weldability in this manner, and is excellent in strength-ductility balance, thereby obtaining good seismic resistance. As a result, it is suitable for use in large structures such as ships, offshore structures, bridges, and building structures.

First, in the high strength thick steel sheet of 590 to 780 MPa class of the present invention, the structure of the main phase is bainite. The bainite structure assures uniform elongation improvement, good strength-ductility balance, good seismic resistance and the like while ensuring good base material toughness and high heat input weldability. For this reason, it is preferable that a thick steel plate is the structure of the bainite principal containing residual (gamma).

However, production or mixing of polygonal ferrite, martensite, or cementite is allowed within the range not to impair the above characteristics other than this bainite due to manufacturing costs or limitations. . However, these tissues are preferably as few as possible because they lower the strength-ductility balance. In particular, polygonal ferrite, which is easily formed or mixed in the cooling process after rolling, has a fraction of 15% or less of the thick steel sheet structure in terms of improving the strength-ductility balance, weld cracking resistance, and high heat input HAZ toughness. desirable.

Regarding the fraction (volume fraction) of metamorphic structures such as polygonal ferrite and bainite, after corrosion of the plated quarter of each steel plate with a 3% nital corrosion solution after surface polishing, The structure is observed with an optical microscope (magnification: 1000 times), the area | region of 50 micrometers is imaged by n = 10, and it measures by image analysis, such as a point counting method.

(Residual γ)

Next, in the present invention, the residual? Regulation in the thick steel sheet structure will be described below. The fraction of residual γ in the thick steel sheet structure of the present invention is set to 1.0 to 30% by volume fraction as a premise of obtaining an excellent strength-ductility balance. If the fraction of residual gamma is less than 1.0%, the "TRIP" effect of residual gamma is not exhibited. As a result, as a premise, in the high strength thick steel sheet of 590 to 780 MPa class, excellent strength-ductility balance of tensile strength x uniform elongation of 14000 or more is not obtained. On the other hand, when the fraction of residual (gamma) exceeds 30%, island-like martensite tends to increase and toughness falls.

(KTP value)

In the present invention, as described above, the amount of C in the residual γ is increased (by increasing the C concentration) to ensure the stability of the residual γ. Thereby, the balance (amount) of the residual (gamma) and the stability of the residual (gamma) can be balanced, and uniform elongation can be improved. As a result, in the high strength thick steel sheet of 590 to 780 MPa class, excellent strength-ductility balance of 14000 or more in tensile strength 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, and the larger the amount of Mn, Cr, Ni, Cu, etc., the following Ms [ Ms [γ _R ], which is the Ms point of residual γ, is lowered as in the formula γ _R ], and the residual γ is stabilized.

And, as shown in the following KTP value equation, the lower the Ms [γ _R ] and the higher the γ _R fraction, the higher the O is the reciprocal of the Ms [γ _R ] and the γ _R fraction becomes larger than O. All. Therefore, it can be said that the following KTP value is an index of stability of residual gamma, or an index of transformation organic plasticity ("TRIP") effect of austenite, and an index of strength-ductility balance. The fraction (amount) of residual γ and the stability of residual γ can be balanced.

On the other hand, if the KTP value is less than 0, the amount of C in the residual γ is lowered, Ms [γ _R ], which is the Ms point of the residual γ, rises and the residual γ becomes unstable. For this reason, the strength-ductility balance falls.

KTP value = -3.14 x 10 ³ + 163 x [γ _R fraction] + 5.09 × 10 ⁵ × (1 / Ms [γ _R ]) ≥ 0

However, Ms [γ _R ] is the Ms point (martensite transformation start temperature) of residual γ

Ms [γ _R ] = 550-361 [% C (γ _R )]-39 [% Mn] -20 [% Cr] -17 [% Ni] -10 [% Cu] -5 [% Mo] .

However,% C (γ _R ) is the amount of C in the residual γ.

The expression itself of this KTP value is used to determine the amount of residual γ and the stability of residual γ (the Ms point of the residual γ) in order to quantitatively evaluate the amount of residual γ (γ _R ) and the effect that the stability of the residual γ has on the reduction of "TRIP". Several samples with different values) were prepared, and a regression equation was prepared from TS (tensile strength) × EL (uniform elongation) obtained from these samples and each of the above parameters to obtain TS x EL = 14000 (strength). -The formula is normalized so that the critical condition of the ductility balance is zero.

(Measurement of Residual γ)

The fraction of the said residual gamma can be measured from the X-ray-diffraction measurement of a steel plate structure with respect to the 1/4 plate | board thickness of the steel plate. That is, for example, using an X-ray diffraction measuring apparatus (RAD-RU300 manufactured by Rigaku Denki Co., Ltd.), the target is Co and the target output is 40 kV-200 mA, and the X-ray diffraction peak of the steel sheet structure is obtained. The theoretical intensity ratio is calculated | required by the Bert method by calculation, and residual amount (gamma amount) is measured.

In addition,% C (γ _R ), which is the amount of C (C concentration) in the residual γ, is coated with Si as a standard substance on a steel sheet sample, and the X-ray diffraction measurement is performed to determine Si and X-rays of residual γ (γ _R ). Determine the diffraction peak position. Using this peak position, the lattice constant a _O of the residual γ is measured. Use peaks shall be (111), (200), (220), and (311).

And from this lattice constant a ₀ , C amount (C concentration) in residual (gamma) by the formula of the following formula described in "DJ Dyson et al., Journal of The Iron and Steel Institute, (1970) p 469-474". Obtain The following formula calculates another carbide-forming element contained in the steel sheet as a negative factor with respect to the amount of C in the residual γ.

_{% C (γ R) = (} a 0 - 3.578 - 0.00095 ×% Mn + O.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: mass%) of the thick steel sheet of this invention is demonstrated below including the reason for limitation of each element. The composition of the thick steel sheet of the present invention is provided on the premise of controlling the structure of the thick steel sheet of the present invention to ensure good base material toughness and high heat input weldability while improving uniform elongation to obtain excellent strength-ductility balance and good seismic resistance. It is effective to manufacture by the method prescribed | regulated in the range shown below.

Namely, by mass%, C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 1.5 to 7.0%, Al: 0.1% or less (not including 0%), Ti: 0.002 to 0.1%, N: 0.001 To 0.01%, respectively, and the balance is substantially steel and inevitable impurities.

Hereinafter, the reason which prescribed | regulated each element amount is explained in full detail.

C: 0.01 to 0.1%.

C (carbon) exerts a "TRIP" effect, increases the amount of C in the residual γ (by increasing the C concentration), ensures the stability of the residual γ, and makes the KTP value above 0, and class 590 to 780 MPa. In the high strength thick steel sheet, it is an important element in order to secure an excellent strength-ductility balance of tensile strength x uniform elongation of 14000 or more. In addition, C is also effective for ensuring the weld cracking resistance of the HAZ portion during welding, the high heat input HAZ toughness, and the strength of the base metal.

In order to exert such an effect, at least 0.01% is required, and when 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 Ms [γ, which is the Ms point of the residual γ. _R ] rises, the remaining γ becomes unstable, and the KTP value becomes less than zero. For this reason, in the high strength thick steel sheet of 590-780 MPa grade, the outstanding strength-ductility balance whose tensile strength x uniform elongation is 14000 or more cannot be ensured.

On the other hand, when the amount of C exceeds 0.10% and becomes excessive, martensite rather than low-temperature transformation bainite is produced on the high cooling rate side, so that the crack resistance and the high heat input HAZ toughness are not improved. Therefore, C content is 0.01 to 0.10%, Preferably it is 0.02 to 0.08% of range.

Si: 0.05-2.0%.

Si has an inhibitory effect of cementite formation and improves strength-ductility balance. In addition, by strengthening employment, it contributes to securing the strength of the base material. This effect is exhibited with 0.05% or more, Preferably it contains 0.2% or more. On the other hand, when it contains excessively exceeding 2.0%, base material toughness and HAZ toughness will fall together. For this reason, Si content is 0.05 to 2.0%, Preferably you may be 0.2 to 2.0% of range.

Mn: 1.5-7.0%.

Mn has the effect of improving the hardenability of the steel and at the same time makes it easy to produce low-temperature transformation bainite at high to low cooling rates. If the Mn content is less than 1.0%, the desired hardenability improving effect is not exerted, the residual γ is destabilized and the base material strength is also insufficient, resulting in a decrease in the strength-ductility balance. On the other hand, when it contains exceeding 7.0% excessively, HAZ toughness will deteriorate. Therefore, the range of Mn content is 1.5 to 7.0% of range, Preferably it is 2.0 to 60% of range.

Al: 0.1% or less (does not contain 0%).

Al fixes the dissolved nitrogen as AlN, and also increases the strength-ductility balance by strengthening the solid solution. On the other hand, when Al is contained in excess of 0.1% excessively, solid solution strengthening will become excessive and it will reduce base material properties, such as toughness. For this reason, Al content is decided by nitrogen amount, and when there is no nitrogen, it does not need to contain especially. Therefore, Al content is 0.1% or less (0% is not included), Preferably it is 0.05% or less (0% is not included).

Ti (total amount): 0.002 to 0.1%.

Ti is useful in that it forms nitrogen and nitride, or oxygen and oxide, and contributes to the improvement of the HAZ toughness by miniaturizing γ grains of the HAZ portion during high heat input welding. In order to exhibit such an effect effectively, it is made to contain 0.002% or more in Ti (total amount). On the other hand, when Ti amount exceeds 0.1% by Ti (total amount), Ti nitride or Ti oxide becomes excessive or coarse, and deteriorates both HAZ toughness and base metal toughness. Therefore, the total Ti content (total amount) is in the range of 0.002 to 0.1%, preferably 0.005 to 0.05%.

N: 0.001-0.01%.

N (nitrogen) is useful in that Ti forms nitrogen and nitride, or oxygen and oxide, and contributes to the improvement of HAZ toughness by miniaturizing γ grains of the HAZ portion during high heat input welding. In order to exhibit these effects effectively, it is contained 0.001% or more. On the other hand, when the amount of N exceeds 0.01% excessively, the base metal toughness and the HAZ toughness deteriorate together. Therefore, N content is 0.001 to 0.01%, Preferably it is 0.0030 to 0.0080% of range.

Below, the element to selectively contain is demonstrated.

Any one or two or more of Cr, Ni, Cu, and Mo.

Cr, Ni, Cu, and Mo together become a negative term in the formula of Ms [γ _R ] in the KTP value, stabilize the residual γ, and enhance the strength-ductility balance. When this effect is exerted, any one or two or more of Cr, Ni, Cu and Mo are selectively contained in a total of 0.2% or more.

On the other hand, when one or two or more kinds of these elements are excessively contained in excess of 5% in total, the remaining γ becomes excessively stable, and thus the "TRIP" effect cannot be obtained. Therefore, when selectively containing any 1 type, or 2 or more types of Cr, Ni, Cu, and Mo, it is 0.2-5% in total, Preferably it is the range of 0.5-3.0% in total.

B: 0.0005 to 0.0050%.

B suppresses the formation of polygonal ferrite in the cooling process after rolling, and secures the base material strength. In addition, it is effective to refine the steel structure to improve the base material toughness. This effect is exhibited at 0.0005% or more. On the other hand, when B content exceeds 0.0050%, these effects are saturated. Therefore, B content is taken as 0.0005 to 0.0050% of range.

Any one or two or more of Nb, V, Zr, and W;

Nb, V, Zr, and W form carbides (MC) to increase the base material strength. When this effect is exerted, any one or two or more of Nb, V, Zr and W are selectively contained in 0.01% or more in total.

On the other hand, when one or two or more kinds of these elements are excessively contained in excess of 0.5% in total, MC is increased, free carbon in steel is reduced, and stability of residual? Is reduced. Therefore, when selectively containing any 1 type, or 2 or more types of Nb, V, Zr, and W, it is made into the range of 0.01 to 0.5% in total.

REM: 0.001-0.1%.

REM refines inclusions such as sulfides such as MnS to improve HAZ toughness. In the case of exhibiting this effect, the content is optionally 0.001% or more. However, REM saturates even above 0.1%. Therefore, when REM is selectively contained, it is in the range of 0.001 to 0.1%.

Next, unavoidable impurities will be described below. Elements other than the above are impurities and allow inclusion in a range that does not impair thick steel sheet characteristics. For example, P (phosphorus) and S (sulfur) are also elements that exist as unavoidable impurities, and have adverse effects such as lowering weldability and base metal toughness. Therefore, it is preferable to suppress P to 0.020% or less and S to 0.010% or less.

(Production method)

The thick steel sheet of this invention includes hot rolling, and the process itself can be manufactured by a conventional method. That is, it is melted by the usual solvent methods, such as a converter, and is made into steel raw material (slab) of predetermined dimension by normal casting methods, such as a continuous casting method. The steel material (slab) is made of a structure that is recrystallized at the end of hot rolling, by performing hot rolling after heating, inhibiting the development of the aggregate structure along the rolling direction, according to the usual thick steel sheet production method. The steel sheet after completion of hot rolling is quenched with water. Thereafter, the steel sheet is tempered to obtain a product thick steel sheet.

Although rolling conditions are not specifically limited, Preferably, after heating to 1000-1200 degreeC, it rolls by making finish rolling temperature into 700-900 degreeC. By such low-temperature rolling, the structure | tissue after heat processing in the two phase area mentioned later can be refined | miniaturized, and characteristics, such as a base material toughness, can be improved.

Moreover, it is preferable to cool the cooling rate in forced cooling, such as water quenching after rolling, at 1.0-20 degreeC / s. By increasing the cooling rate in this way and increasing the fraction of bainite, the structure after heat treatment in the two-phase region described later can be refined, and properties such as base material toughness can be improved.

In order to satisfy the conditions of the structure, in particular, attention is paid to the temperature conditions during the heat treatment (tempering, tempering). The temperature of the heat treatment is overheated in the α + γ two-phase region between Ae1 and Ae3. Specifically, the temperature of the heat treatment (heating temperature: T _hold : ° C.) is heated and maintained so that (T _hold −Ae1) / (Ae3-Ae1) × 100 (%) is in a range of 5 to 50%. By maintaining at this temperature, fine austenite can be formed, and after that, when cooled to room temperature, it can be set as the bainite structure which formed 1.0-30% of residual (gamma) in a volume fraction. On the other hand, the holding time is a time sufficient to obtain these effects, but it is preferable to hold the holding time for 3 minutes or more.

Heating temperature: When T _hold becomes less than 5% in (T _hold -Ae1) / (Ae3-Ae1) x 100 and becomes an alpha + θ region of less than Ae1, when cooled to room temperature, it is 1.0% by volume fraction. Sufficient amount of residual γ above cannot be secured.

On the other hand, when the temperature of the heat treatment exceeds 50% in (T _hold -Ae1) / (Ae3-Ae1) x 100 and becomes γ in the region beyond Ae3, when the temperature of the heat treatment is cooled to room temperature, the residual γ is 30 by volume fraction. Since the concentration of carbon in the residual γ cannot be sufficiently exceeded by%, the stability of the residual γ is lowered, and a sufficient amount of the residual γ cannot be secured.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by the following example, of course, It is not limited to the following example, It is also carried out by changing suitably to the range suitable for the purpose of the former and the later. Of course, they are possible and all are included in the technical scope of this invention.

<Example>

150 kg ingot was produced by vacuum-dissolving the steel of the chemical composition shown in Table 1 (Invention Examples A-P and Comparative Examples Q-W). This ingot was subjected to multipass rolling and forced cooling under the rolling conditions shown in Table 2 to obtain a steel plate having a sheet thickness of 30 mm. The steel sheet was subjected to heat treatment (heating time was approximately 1 hour in common) under the two-phase reverse heat treatment conditions shown in Tables 2 and 3 to obtain test specimens. In Table 1, Ae1 and Ae3 of each steel sheet are calculated using Thermocalc, which is thermodynamic software.

In addition, in the heating conditions of the two-phase heat treatment of Table 2, the conditional expression of the heating temperature (T _hold : ° C) in this two-phase heat treatment shown in Table 3, (T _hold -Ae1) / (Ae3-Ae1) x 100 (%) ) Is described along with the error range (±%) based on the error of the heating temperature.

A sample was taken from the steel sheet thus obtained, and as shown in Table 3, in the thick steel sheet structure, the volume fraction (α fraction:%) of polygonal ferrite, the fraction of residual γ (γ _R fraction:%), and the C in residual γ amount [C (γ _R):%], Ms point (Ms γ _[R]: ℃) of the residual γ, was determined by the aforementioned method of measuring the value of each KTP or calculated. In addition, it was also confirmed whether the structure of the balance was a bainite subject.

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

(Base material tensile properties)

A JIS 4A test piece was taken from the sample, a tensile test according to JIS Z2241 was performed, and the tensile strength (TS: MPa) and the uniform elongation (EL: strain) when the load decreased by 5% from the maximum value (5%). In addition, the strength-ductility balance of TS x EL was also determined, and the strength-ductility balance (MPa%) was evaluated to be 14000 or more.

(Base metal toughness)

The Charpy test piece was cut out from the depth part of plate | board thickness t / 4, the Charpy impact test based on JISZ2242 was done, and the toughness (vE0: J) at ₀ degreeC was measured. And if you have more than vE ₀ 11OJ rated it excellent toughness base.

(Welding seam toughness)

A heat cycle (5 cm in size, 12.5 mm x 32 mm x 55 mm) cut out from the sample, heated at 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 with kJ / mm heat input). The Charpy test piece was extract | collected from each of these test pieces, the Charpy impact test based on JISZ2242 was done, and the toughness (vE0: J) at ₀ degreeC was measured. And if vE ₀ 100 J or more it was evaluated to be superior toughness of the weld seams.

As is clear from Tables 1 to 3, Inventive Examples 1 to 17 use the steels of Inventive Examples A to P of Table 1 satisfying the composition of the present invention, and the preferable two-phase heat treatment of 1 and 2 in Table 2 It is manufactured in the manufacturing conditions. For this reason, the fraction of residual gamma in a thick steel plate structure is 1.0 to 30% of range, and this fraction of residual gamma satisfies the following KTP value. Further, Examples 1 to 17 of the steel plate, organizations, γ _R fraction shown in Table 3 (percentage of residual γ) and, α fraction glass except for (poly fraction of goneol ferrite) parts organization that bainite, the bainite as a main component It was.

As a result, the strength-ductility balance of 14000 MPa% or more is obtained in the high strength thick steel sheet of 590 MPa grade or more. In addition, the base material toughness is excellent. In addition, the thermal cycle characteristic also has a toughness of 100J or more, and is excellent in weldability such as weld seam toughness.

These results show that when used as a high strength 590-780 MPa grade steel plate for a building structure or a steel structure, favorable seismic resistance is obtained.

On the other hand, the comparative examples 18-24 use the steel of the comparative examples Q-W of Table 1 in which the component composition of any element deviates from the invention range. For this reason, although the manufacturing conditions, such as rolling and a two-phase reverse heat processing, are also manufactured in a preferable range, even if the structure regulation, such as the fraction of residual (gamma), falls outside or enters, the strength-ductility balance of 14000 MPa% or more, Any of the heat cycle characteristics or all the characteristics are inferior to the invention examples.

In addition, in Comparative Example 25, regardless of whether the steel of Inventive Example A in Table 1 that satisfies the present invention composition is used, manufacturing conditions such as rolling and two-phase reverse heat treatment are produced outside a preferable range, and the fraction of residual? Deviation from the organizational rules, either the strength-ductility balance of 14000 MPa% or more, the heat cycle characteristics, or both, are inferior to the inventive examples.

As a result, these comparative examples cannot be used as 590-780 MPa grade thick steel sheets for building structures and steel structures which are required to be shockproof.

In Comparative Example 18, the amount of C in steel Q was too high, out of the upper limit. For this reason, although structures, such as the fraction of residual (gamma), exist in the invention range, heat cycle characteristics are low and it is inferior to weldability.

In Comparative Example 19, the amount of Si in the steel R was too high to deviate from the upper limit. For this reason, although structures, such as the fraction of residual (gamma), exist in the invention range, heat cycle characteristics are low and it is inferior to weldability.

In Comparative Example 20, the amount of Si in the steel S was too low to deviate from the lower limit. For this reason, there is little residual γ, and the fraction of 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 was too low to be out of 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 amount of Mn in the steel U was too high to exceed the upper limit. For this reason, although structures, such as the fraction of residual (gamma), exist in the invention range, heat cycle characteristics are low and it is inferior to weldability.

In Comparative Example 23, the amount of Al in the steel V was too high, exceeding the upper limit. For this reason, the amount (alpha fraction) of polygonal ferrite increases, and the fraction of residual (gamma) does not satisfy a KTP value, but is inferior in intensity-ductility balance and thermal cycling characteristics.

In Comparative Example 24, the amount of Ti in the steel W was too high, out of the upper limit. Thus, carbides of Ti are formed, and no residual γ is formed, and the strength-ductility balance and heat cycle characteristics are inferior.

Comparative Example 25 is produced by a two-phase heat treatment outside the preferred range of Table 2, although the steel of Inventive Example A in Table 1 satisfying the composition of the present invention is used. For this reason, since the fraction of residual (gamma) is too small, it does not satisfy | fill KTP value, and is inferior in intensity-ductility balance and heat cycling characteristics together.

From the above results, in the case of a high strength 590 to 780 MPa grade thick steel sheet defined by the component composition and structure of the present invention, the critical significance of the strength-ductility balance and the weldability toughness is supported.

As described above, according to the present invention, it is possible to provide a high-strength thick steel sheet of 590 to 780 MPa grade having excellent strength-ductility balance and improving the uniform elongation while ensuring good base material toughness and high heat input weldability, and a method of manufacturing the same. . For this reason, the thick steel plate of this invention can be applied to the structure and building structure for which earthquake resistance is calculated | required.

Claims (6)

% By mass C: 0.01 to 0.10%, Si: 0.05 to 2.0%, Mn: 1.5 to 7.0%, Al: 0.1% or less (not including 0%), Ti: 0.002 to 0.1%, N: 0.001 to 0.01 A thick steel sheet containing% each, the balance being substantially iron and unavoidable impurities, wherein the fraction of residual? In the thick steel sheet structure is 1.0 to 30%, and the fraction of this residual? Satisfies the following KTP value. A thick steel sheet having excellent strength-ductility balance and weldability. KTP value = -3.14 x 10 ³ + 163 x [γ _R fraction] + 5.09 x 10 ⁵ x (1 / M _s [γ _R ]) ≥O. However, Ms [γ _R ] is the Ms point (martensite transformation start temperature) of residual γ, Ms [γ _R ] = 550-361 [% C (γ _R )]-39 [% Mn]-20 [% Cr]-17 [% Ni]-10 [% Cu]-5 [% Mo] . However,% C (γ _R ) is the amount of C in the residual γ. The method of claim 1, Furthermore, the thick steel plate which is excellent in strength-ductility balance and weldability which contains 0.2-5% of any 1 type, or 2 or more types of Cr, Ni, Cu, and Mo in total. The method of claim 1, A thick steel sheet further having a strength-ductility balance and weldability, further comprising B: 0.0005 to 0.0050%. The method of claim 1, Furthermore, the thick steel plate which is excellent in strength-ductility balance and weldability which contains 0.01-0.5% of any 1 type, or 2 or more types of Nb, V, Zr, and W in total. The method of claim 1, Further, a steel sheet having excellent strength-ductility balance and weldability containing 0.001 to 0.1% of REM. The method of claim 1, A thick steel sheet having an excellent strength-ductility balance and weldability, wherein a fraction of the polygonal ferrite of the thick steel sheet structure is 15% or less.