KR100954041B1 - Thick steel plate having excellent toughness of weld heat-affected zone and excellent base material toughness - Google Patents

Abstract

An object of the present invention is to provide a thick steel sheet which exhibits excellent HAZ toughness in both quench heat welding and super heat input welding, and also has excellent base material toughness.

The thick steel sheet is C: 0.030 to 0.10%, Si: 1.0% or less, Mn: 0.8 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.035%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, and N: 0.0050 to 0.01%, old austenite grain size is 120 µm or less, phase martensite (MA) is 3% or less, and aspect ratio of MA is 3 or less And the following formulas 1 and 2 are satisfied.

[Equation 1]

1.5 ≤ [Ti] / [N] ≤ 4

[Equation 2]

40 ≤ X value ≤ 160

X value = 500 [C] + 32 [Si] + 8 [Mn]-9 [Nb] + 14 [Cu] + 17 [Ni]-5 [Cr]-25 [Mo]-34 [V]

HAZ toughness, base material toughness, thick steel plate, X value, heat input

Description

THICK STEEL PLATE HAVING EXCELLENT TOUGHNESS OF WELD HEAT-AFFECTED ZONE AND EXCELLENT BASE MATERIAL TOUGHNESS}

The present invention relates to a thick steel sheet applied to welded structures, such as ships and offshore structures, for example. Preferably, the heat affected zone after welding even if welded with a wide heat input amount from quench heat welding to super heat input welding. The present invention relates to a thick steel sheet having excellent toughness of Affected Zone (HAZ) and excellent base material toughness.

In recent years, for example, enlargement of a container ship etc. advances, and the thick steel plate whose plate | board thickness is 60 mm or more may be used. In order to weld such a thick steel plate efficiently, it is required to be excellent in HAZ toughness not only at the time of quench heat welding but also at the time of super heat input welding of 50 kJ / mm or more in heat input.

However, when the super heat input welding is performed, since the HAZ is heated to a high temperature austenite region and then cooled slowly, there is a problem that the structure is coarsened and the HAZ toughness is significantly degraded. Therefore, conventionally, the welding heat input amount was inevitably limited.

In order to achieve good HAZ toughness in such superheat input welding, for example, Patent Document 1 controls the addition balance of Nb and B in addition to lower C and lower P. Moreover, in patent document 2, Nb is actively contained in TiN type interference | inclusion existing in welding steel, and generation | occurrence | production of coarse ferrite is suppressed. However, in these patent documents 1 to 2, when TiN is insufficient or TiN is satisfied, the TiN is coarsened, and there is room for further improvement of HAZ toughness. Neither was it considered for base metal toughness.

Patent document 3 says that by adding a relatively large amount of N to the steel, and properly controlling the balance of addition of Ti and B, the HAZ toughness at the time of high heat input welding can be improved. However, also in Patent Document 3, since the amount of precipitation of TiN and BN is not sufficient, fine, or Nb is insufficient and the hardenability is low, the ferrite becomes coarse, so there is room for further improvement of HAZ toughness. It also does not consider the base material toughness.

[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-166033

[Patent Document 2] Japanese Patent Application Laid-Open No. 2004-218010

[Patent Document 3] Japanese Patent Application Laid-Open No. 2005-200716

An object of the present invention is to provide a thick steel sheet which exhibits excellent HAZ toughness in both quench heat welding and super heat input welding, and also has excellent base material toughness.

The thick steel plate excellent in the HAZ toughness and base material toughness which concerns on the said subject which could solve the said subject is C: 0.030 to 0.10% (mean of mass%, the same below), Si: 1.0% or less (0% is not included) ), Mn: 0.8% to 2.0%, P: 0.03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Nb: 0.005 to 0.035% , Ti: 0.015% to 0.03%, B: 0.0010% to 0.0035%, and N: 0.0050% to 0.01%,

Cu: 2.0% or less (including 0%), Ni: 2.0% or less (including 0%), Cr: 1% or less (including 0%), Mo: 0.5% or less (0% And V: 0.1% or less (including 0%),

The remainder is a thick steel plate made of Fe and unavoidable impurities,

The average equivalent circle diameter of the old austenite grain is 120 µm or less, the area percentage of island-like martensite (MA) is 3% or less, and the aspect ratio (long diameter) of the island-like martensite (MA) / Short diameter) has a number average of 3 or less,

Furthermore, it has the point at the point which satisfy | fills following formula (1) and formula (2).

[Equation 1]

1.5 ≤ [Ti] / [N] ≤ 4

[Equation 2]

40 ≤ X value ≤ 160

X value = 500 [C] + 32 [Si] + 8 [Mn]-9 [Nb] + 14 [Cu] + 17 [Ni]-5 [Cr]-25 [Mo]-34 [V]

[Ti], [N], [C], [Si], [Mn], [Nb], [Cu], [Ni], [Cr], [Mo], and [V] in the formula Content (mass%) of each element]

The temperature range of the delta region of the thick steel plate of this invention is 40 degrees C or less, for example. In the thick steel plate of this invention, it is preferable that the average particle diameter of Ti type carbonitride is 40 nm or less in the position (t = plate thickness) of depth t / 4.

The thick steel sheet of the present invention further contains Ca: 0.005% or less (does not contain 0%), Mg: 0.005% or less (does not contain 0%), REM: 0.01% or less (does not contain 0%), Zr: 0.1% or less (without 0%), Hf: 0.05% or less (without 0%), Co: 2.5% or less (without 0%), W: 2.5% or less (0% May not be included).

In addition, in this specification, "carbonitride" is used by the meaning containing also carbide and nitride.

According to the present invention, not only the amount of each element is individually controlled, but also the mutual relationship between the amount of each element is controlled from the viewpoint of X value, Ti / N ratio, etc. Since the size, shape, and the like of the in-phase martensite MA are controlled, a thick steel sheet exhibiting excellent HAZ toughness and excellent base metal toughness can be obtained in both of the heat input welding and the super heat input welding.

In the thick steel sheet of the present invention, in order to improve the HAZ toughness, the (A) X value and (B) Ti / N ratio are controlled, and the ductility and brittle transition temperature [waveform rate transition temperature (vTrs). Hereinafter, simply referred to as transition temperature], the size and shape of the (C) spherical austenite grains and the phase martensite structure (MA) are controlled to improve the base material toughness. Hereinafter, it demonstrates in order.

(A) X value

The X value is a function of the temperature range in the δ range. An attempt is made to improve the HAZ toughness and explain how the X value is reached. First, the present inventors attempted to achieve good HAZ toughness even in super heat input welding by refine | purifying Ti type carbonitride. The conventional dispersion state of Ti-based carbonitride has been considered to be determined only by the addition balance of Ti and N as long as the cooling rate at the time of molten steel coagulation is constant. However, as a result of earnestly examining by the present inventors, it was found that by reducing the temperature range of the region δ shown in the state diagram of the steel, Ti-based carbonitride can be finely dispersed even with the same Ti and N addition amount.

Said "delta region" means the area | region where (delta) iron is contained in the state diagram of steel. This "region in which δ iron is contained" includes not only the region of δ iron but also a region in which a state different from δ iron is included, such as a two-phase region of δ + γ. The term " temperature range in the δ " refers to a temperature range in which δ iron is contained (the difference between the upper limit temperature and the lower limit temperature in the δ region). In the steel of a specific composition, for example, when there is a temperature range of only δ iron and a temperature range of δ + γ iron, the sum of these temperature ranges is a temperature range in the δ range. The temperature range of this delta region can be calculated by inputting the chemical composition of a steel plate into total thermodynamic calculation software (Thermo-calc, which can be purchased from CRC total laboratory).

In this δ iron, the diffusion rate of Ti is fast. It is considered that the wider the temperature range in the δ region, the longer the time that δ iron is present and the diffusion of Ti proceeds, so that coarse Ti-based carbonitrides are more likely to be formed. Therefore, the refinement | miniaturization of Ti type carbonitride was examined by adjusting chemical composition and reducing the temperature range of (delta) region. The influence on the temperature range of the δ region of each chemical component was examined by repeating the calculation of Thermo-calc while changing only one of the chemical components based on the specific component. Based on this calculation, an X value (Equation 3 below) was determined that correlates with the temperature range in the δ range and is expressed as a function of the chemical composition:

[Equation 3]

X value = 500 [C] + 32 [Si] + 8 [Mn]-9 [Nb] + 14 [Cu] + 17 [Ni]-5 [Cr]-25 [Mo]-34 [V]

[In formula, [C], [Si], [Mn], [Nb], [Cu], [Ni], [Cr], [Mo], and [V] are content (mass%) of each element in a steel plate. Indicates

The coefficient in Formula 3, which determines the X value, corresponds to the amount of decrease in the temperature range in the δ range when each chemical component is changed from the steel of the specific component. Specifically, for example, "500" of the coefficient of [C] means that when the amount of C is increased by 0.01%, the temperature range in the δ range decreases by about 5 ° C by calculation of Thermo-calc. The temperature range between the X value and the δ range is in inverse proportion (relative to the fact that when the X value increases, the temperature range in the δ range decreases).

As a result of manufacturing a steel sheet having various X values and examining their properties, the Ti-based carbonitride was refined by increasing the X value (by narrowing the temperature range in the δ range), and furthermore, either quench heat welding or superheat input welding. HAZ toughness was also improved.

Therefore, in the thick steel sheet of the present invention, the X value satisfies the following expression (2). In addition, although the meaning of X value is interpreted as mentioned above, the most important thing is that there exists a correlation between X value and various characteristics, and satisfying X value irrespective of interpretation is contained in this invention.

[Equation 2]

40 ≤ X value ≤ 160

The range of X value is 40 or more, Preferably it is 45 or more, More preferably, it is 50 or more. As the X value increases, the Ti-based carbonitride becomes finer, and the HAZ toughness becomes better. However, as the X value increases, the martensite-austenite constituent (MA) increases. Therefore, X value is 160 or less, Preferably it is 100 or less, More preferably, it is 75 or less.

(B) Ti / N ratio

In the thick steel sheet of the present invention, the HAZ toughness is improved by balancing the Ti amount and the N amount. Specifically, the following formula 1 is satisfied.

[Equation 4]

1.5 ≤ [Ti] / [N] ≤ 4

[In formula, [Ti] and [N] represent content (mass%) of each element in a steel plate.]

When [Ti] / [N] exceeds 4, the Ti-based carbonitride becomes coarse and HAZ toughness is lowered. Preferable [Ti] / [N] is 3.5 or less. On the contrary, when [Ti] / [N] is less than 1.5, HAZ toughness is lowered under the influence of excess N. Preferable [Ti] / [N] is 2.0 or more, More preferably, it is 2.5 or more.

From the viewpoint of toughness, the Ti-based carbonitride in the thick steel sheet of the present invention is preferably fine. The Ti-based carbonitride in the thick steel sheet of the present invention is, for example, 40 nm or less, preferably 30 nm or less.

In addition, the value of the average particle diameter of Ti type carbonitride in this invention is the value measured as follows. First, the position (t = plate thickness) of depth t / 4 as a part representing the thermal history of the steel sheet is observed by a transmission electron microscope (TEM), and the observation magnification is 60,000 times or more (60,000 times in the example described later) and the viewing field It observes on conditions of 2.0 * 2.0 micrometer or more (2.0 * 2.0 micrometer in the Example mentioned later) and 5 or more places of observation sites (5 places in the Example mentioned later). And the area of each carbonitride in the visual field is measured, and the circle equivalent diameter of each carbonitride is computed from this area. The value obtained by carrying out the arithmetic mean (adding each other) of the circle equivalent diameter of each carbonitride is made into the average particle diameter of Ti type carbonitride in this invention.

In addition, determination of whether it is Ti type carbonitride is decided by the component used as a main body of each carbonitride particle. That is, Ti-based carbonitride means that the ratio of Ti becomes 50 mass% or more when the total mass of remaining elements except carbon and nitrogen is 100%. The amount of the element can be determined by an energy dispersive X-ray detector (EDX). In addition, since too fine carbonitride cannot be measured, the carbonitride in this invention is limited to that whose circle equivalent diameter is 5 nm or more.

(C) Old austenite grains and island martensite tissue (MA)

Further, in the thick steel sheet of the present invention, structure control is also performed, specifically, the average circle equivalent diameter of the old austenite grain is 120 µm or less, the area percentage of the island martensite (MA) is 3% or less, and the island martensite ( The number average value of aspect ratio (long diameter / short diameter) of MA) is three or less. By controlling tissue, the transition temperature vTrs can be lowered. In addition, the structure of the thick steel sheet of the present invention is a structure mainly composed of bainite or a structure mainly composed of ferrite and bainite. The main body is said to be 70% or more by area ratio, and the remaining structures include one or two or more of pearlite, martensite, residual austenite, cement and the like, in addition to the above-described island martensite (MA).

However, it is known to improve the base material toughness by making old austenite grains finer. In order to refine old austenite grains, low temperature rolling is generally performed. However, low temperature rolling produces martensite MA which is elongated (large aspect ratio) after transformation. If the aspect ratio of the martensite MA is large, stress is concentrated at the tip portion when the impact is applied, and the toughness deteriorates. On the other hand, in order to reduce the aspect ratio of island martensite MA, rolling at high temperature is considered. However, if hot rolling is carried out, old austenite grains will coarsen at this time, and base metal toughness will deteriorate. For these reasons, it has been difficult to attain both the miniaturization of the old austenite grains and the spheroidization of the phase martensite (MA). In this invention, since the specific hot rolling method mentioned later is employ | adopted, refinement | miniaturization of old austenite grain and spheroidization of phase martensite (MA) are compatible.

The average circle equivalent diameter of the old austenite grains is preferably 110 µm or less, more preferably 100 µm or less. Moreover, although it is not necessary to set the lower limit of an average circle equivalent diameter, the range which can be easily achieved is preferable, for example, 30 micrometers or more, Preferably it may be 60 micrometers or more.

In addition, the measuring method of the average circle equivalent diameter of old austenite grain is as follows. The steel sheet is cut along the rolling direction, and after nitriding the t / 4 (t = plate thickness) position of the cut surface, an optical micrograph (observation magnification: 100 times, observation field: 600 × 800 μm) is taken. (n number = 10). By processing the photographed picture with an image analysis device (Image-Pro Plus, manufactured by Media Cybernetics), an average circle equivalent diameter (mu m) is obtained.

The area percentage of island martensite (MA) is preferably 2.8% or less, more preferably 2.5% or less, and in the most excellent case, it may be 2.0% or less (particularly 1.5% or less). Although it is not necessary to set the lower limit of the area percentage, a range that can be easily achieved is preferable, and may be, for example, 0 or more, preferably 0.5 or more.

The aspect ratio of the island martensite MA is preferably 2.9 or less, and in the most excellent case, it may be 2.5 or less (particularly 2.0 or less). Although it is not necessary to set the lower limit of the aspect ratio, a range that can be easily achieved is preferable, for example, one or more, preferably 1.5 or more.

The measuring method of area percentage of aspect martensite MA and aspect ratio (number average value) is as follows. After nitriding the t / 4 (t = plate thickness) position of the steel plate, an optical micrograph (observation magnification: 1000 times, observation field: 60 × 80 μm) is taken (n number = 10). The area percentage and the aspect ratio (number average) are obtained by processing the photographed picture with an image analysis device (Image-Pro Plus, manufactured by Media Cybernetics).

As described above, in the present invention, the HAZ toughness is improved by controlling (A) X value, (B) Ti / N ratio, (C) spherical austenite grains and phase martensite structure (MA), and the transition temperature ( vTrs). However, in order to exhibit these effects effectively, the component composition of the steel sheet is also important. The component composition of the steel plate of this invention and the reason for limitation are as follows.

[C: 0.030 to 0.10%]

C is an element necessary for securing the strength of the steel sheet, and is an effective element for reducing the temperature range in the region δ in the state diagram of the steel. If the amount of C is less than 0.030%, the strength cannot be secured. On the other hand, when C amount exceeds 0.10%, hard 2nd phase structure MA will become large too much, and base material toughness, transition temperature vTrs, and HAZ toughness will deteriorate. Therefore, the amount of C was set to 0.030 to 0.10%. The minimum with preferable C amount is 0.04% or more, More preferably, it is 0.05% or more. Moreover, the upper limit with preferable C amount is 0.09% or less, More preferably, it is 0.07% or less.

[Si: 1.0% or less (not including 0%)]

Si is an effective element for securing the strength of the steel sheet, and for that purpose, Si is preferably added at least 0.01%. However, when Si is added too much, since many MA structures will generate | occur | produce, and a base material toughness and HAZ toughness will fall, it is necessary to make the upper limit into 1.0%. The minimum with preferable Si amount is 0.05% or more, More preferably, it is 0.1% or more. The upper limit with preferable Si is 0.8% or less, More preferably, it is 0.5% or less.

[Mn: 0.8% to 2.0%]

Mn is an element effective in improving the hardenability and securing the strength of the steel sheet. If the amount of Mn is less than 0.8%, the effect of securing strength is not sufficiently exhibited. On the other hand, when Mn amount exceeds 2.0%, base material toughness and HAZ toughness will fall. Therefore, Mn amount was set to 0.8 to 2.0%. The minimum with preferable Mn amount is 1.0% or more, More preferably, it is 1.2% or more. On the other hand, the upper limit with preferable Mn amount is 1.8% or less, More preferably, it is 1.6% or less.

[P: 0.03% or less (not including 0%)]

Since the impurity element P adversely affects the base material toughness and the HAZ toughness, the amount is preferably as small as possible. Therefore, the amount of P is 0.03% or less, Preferably it is 0.02%. However, industrially, it is difficult to make P amount in steel into 0%.

[S: 0.01% or less (not including 0%)]

S is an element that forms MnS and lowers the ductility. In particular, the amount of S is preferably as small as possible because the adverse effect of the high tensile strength steel is increased. Therefore, the amount of S is 0.01% or less, Preferably it is 0.005% or less. However, industrially, it is difficult to make S amount in steel 0%.

[Al: 0.01% to 0.10%]

Al is an element having an effect of improving base metal toughness by deoxidation and micronization of microstructure. In order to fully exhibit such an effect, Al is added 0.01% or more. However, when Al is excessively added, the base metal toughness and the HAZ toughness are lowered, so the upper limit is made 0.10%. The minimum with preferable Al amount is 0.02% or more. On the other hand, the upper limit is preferably 0.06% or less, and more preferably 0.04% or less.

[Nb: 0.005% to 0.035%]

Nb is an effective element for improving the hardenability of the base and increasing the strength of the steel sheet. In order to fully exhibit such an effect, Nb amount needs to be 0.005% or more. However, when Nb is added excessively, since the base metal toughness and HAZ toughness fall, the upper limit was set to 0.035%. Nb amount becomes like this. Preferably it is 0.010% or more, Preferably it is 0.025% or less, More preferably, it is 0.020% or less.

[Ti: 0.015% to 0.03%]

Ti is an effective element for forming fine nitrides with N and improving HAZ toughness (by the so-called pinning effect) by suppressing coarsening of austenite grains of HAZ during welding. In order to fully exhibit such an effect, Ti is added 0.015% or more. However, when the Ti amount is excessive, the HAZ toughness deteriorates rather, so the upper limit of the Ti amount is set at 0.03%. Ti amount is preferably 0.017% or more (particularly 0.020% or more) and 0.025% or less.

[B: 0.0010% to 0.0035%]

B is an element important for improving HAZ toughness while producing intragranular ferrite having BN as a nucleus in the vicinity of the HAZ, particularly the bond portion, during the superheat input welding, and also has a fixing action of solid solution N. In this invention, in order to fully exhibit the effect, B is contained 0.0010% or more more than content in a normal thick steel plate. However, if the amount of B is excessive, coarse bainite structure is formed at the time of superheat input welding, and therefore HAZ toughness deteriorates. Therefore, the upper limit of the amount of B was set to 0.0035%. The amount of B is preferably 0.0015% or more (particularly 0.0020% or more) and 0.0030% or less (particularly 0.0025% or less).

[N: 0.0050% to 0.01%]

N is an element having the effect of bonding fine Ti to form fine carbonitrides, suppressing coarsening of austenite grains during superheat input welding, and improving HAZ toughness. When the amount of N is too small, since the said effect is not fully exhibited, the minimum was set to 0.0050% or more. On the other hand, when N amount is excessive, since it will adversely affect base material toughness and HAZ toughness, the upper limit was set to 0.01%. The minimum with preferable N amount is 0.006% or more, More preferably, it is 0.007% or more. Moreover, the upper limit with preferable N amount is 0.009% or less, More preferably, it is 0.008% or less.

Although the thick steel plate of this invention contains each said component as an essential component, you may further contain the additional component as needed. For example, the thick steel sheet of this invention may contain 1st additional components, such as Cu, Ni, Cr, Mo, and V, in the range shown below. Moreover, since it is an arbitrary component, although the lower limit is set to 0%, when adding actively, a lower limit becomes more than 0%. In addition, these Cu, Ni, Cr, Mo, V, etc. may be added independently and may be added in combination of 2 or more type.

[Cu: 2.0% or less (including 0%)]

Cu is an element which contributes to strength improvement by increasing hardenability, and can be added as needed. In addition, similar to C, it is thought that the temperature range in the δ region is reduced to reduce the Ti-based carbonitride. In order to fully exhibit such an effect, Cu amount is preferably 0.1% or more, and more preferably 0.2% or more. However, when the amount of Cu is excessive, since the base metal toughness and HAZ toughness tend to fall, the upper limit was set to 2.0%. Cu amount is preferably 1.0% or less, and more preferably 0.5% or less.

[Ni: 2.0% or less (including 0%)]

Ni, like Cu, is an effective element for increasing the hardenability and contributing to the improvement of strength and reducing the temperature range in the δ range. In order to fully exhibit such an effect, Ni amount is preferably 0.2% or more, and more preferably 0.3% or more. However, when the amount of Ni is excessive, since the base metal toughness and HAZ toughness tend to fall, the upper limit was set to 2.0%. Ni amount becomes like this. Preferably it is 1.0% or less, More preferably, it is 0.5% or less.

[Cr: 1% or less (including 0%)]

Cr, like Cu, is an element that improves the hardenability and contributes to strength improvement, and can be added as necessary. In order to fully exhibit such an effect, Cr amount is preferably 0.2% or more, and more preferably 0.4% or more. However, when Cr amount is excessive, since the base metal toughness and HAZ toughness fall, the upper limit was set to 1%. The upper limit with preferable Cr amount is 0.8%.

[Mo: 0.5% or less (including 0%)]

Mo is an element effective in order to prevent temper brittleness in addition to improving hardenability and improving strength, and may be added as necessary. In order to fully exhibit such an effect, Mo amount is preferably 0.05% or more, and more preferably 0.10% or more. However, when the amount of Mo is excessive, since the base metal toughness and the HAZ toughness deteriorate, the upper limit is set at 0.5%. Mo amount is preferably 0.3% or less.

[V: 0.1% or less (including 0%)]

V is an element having an effect of increasing the hardenability and the temper softening resistance by addition of a small amount, and can be added as necessary. In order to fully exhibit such an effect, V amount is preferably 0.01% or more, more preferably 0.02% or more. However, when V amount is excessive, since the base metal toughness and HAZ toughness deteriorate, the upper limit was set to 0.1%. V amount is preferably 0.05% or less.

In the thick steel plate of this invention, you may further contain the 2nd further component as needed. When adding a 2nd further component, those combinations and addition amount are as follows.

(A) At least one selected from Ca: 0.005% or less (does not contain 0%), Mg: 0.005% or less (does not contain 0%), and REM: 0.01% or less (does not contain 0%) ,

(B) Zr: 0.1% or less (does not contain 0%) and / or Hf: 0.05% or less (does not contain 0%),

(C) Co: 2.5% or less (does not contain 0%) and / or W: 2.5% or less (does not contain 0%).

In addition, said (a), (b), (c) may be implemented any one, and may be performed in combination of 2 or more. The details of (a), (b) and (c) will be described below.

(A) For at least one selected from Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.01% or less

Ca, Mg and REM (rare earth elements) are elements having an effect of improving HAZ toughness. Specifically, Ca and REM have an effect of reducing spheroidization of MnS, in other words, reducing anisotropy by controlling the shape of inclusions, thereby improving HAZ toughness. On the other hand, Mg forms MgO and suppresses coarsening of the austenite grains of HAZ, thereby improving HAZ toughness. In order to fully exhibit such an effect, it is preferable to contain Ca 0.0005% or more, Mg 0.0001% or more, and REM 0.0005% or more in a steel plate. However, when these amounts are excessive, the base metal toughness and the HAZ toughness are deteriorated. Therefore, Ca is 0.005% or less, Mg is 0.005% or less, and REM is set to 0.01% or less. Preferably Ca is 0.003% or less, Mg is 0.0035% or less, and REM is 0.007% or less.

(B) Zr: 0.1% or less and / or Hf: 0.05% or less

Zr and Hf form nitrides similarly to Ti and suppress coarsening of the austenite grains of HAZ during welding, and thus are effective elements for improving HAZ toughness. In order to fully exhibit such an effect, Zr amount is preferably 0.0005% or more, and Hf amount is preferably 0.001% or more. However, when these amounts are excessive, the base metal toughness and the HAZ toughness are lowered. Therefore, when these are contained, the upper limit of the amount of Zr is set to 0.1% and the upper limit of the amount of Hf is set to 0.05%.

(C) Co: 2.5% or less and / or W: 2.5% or less

Co and W are elements having the effect of improving the hardenability and increasing the strength of the steel sheet. In order to fully exhibit such an effect, it is preferable to contain one or both of these in 0.1% or more, respectively. However, when these amounts are excessive, since the base metal toughness and the HAZ toughness deteriorate, the upper limit of these amounts is set at 2.5%, respectively.

In the thick steel sheet of the present invention, the remaining portion may be Fe and unavoidable impurities.

In the thick steel sheet of the present invention, a steel that satisfies the requirements of the chemical component amount, [Ti] / [N], and X value is roughly dissolved by a conventional solvent method, and the molten steel is cooled to a slab, After heating on condition of, it can manufacture by hot rolling by the predetermined method mentioned later, and also after cooling by accelerated cooling. In addition, the steel plate after cooling is tempered as needed.

First, the cooling of molten steel will be described in detail. Since the thick steel sheet of the present invention controls the X value to narrow the temperature range in the δ range, the molten steel is cooled under normal conditions (for example, from 1500 ° C to 1100 ° C). Ti-based carbonitride can be made sufficiently small even if the slab is formed by cooling to a cooling rate of 0.1 to 2.0 deg. C / sec. However, in order to form finer carbonitrides, it is preferable to change the cooling water amount and the cooling method of the casting machine to improve the cooling rate at the time of solidification.

And the most important thing in the manufacturing process of the thick steel plate of this invention is hot rolling conditions and subsequent cooling conditions. In the hot rolling of the present invention, the reduction ratio at a temperature of 850 to 800 ° C. is set to 40% or more (eg, about 40 to 80%), and the time between each pass at 850 to 800 ° C. is controlled to 5 to 10 seconds. do. When substantial rolling is performed at a temperature of 850-800 degreeC, the rolling load in temperature below 800 degreeC can be reduced (for example, the rolling reduction can be made less than 5%), and the phase martensite (MA) of Can prevent kidneys. In addition, when substantial rolling is performed at 850-800 degreeC, normally austenite grains will coarsen normally, but in this invention, since the time between each pass is controlled, old austenite grains can be refined. Even if the time between passes is too short or too long, the old austenite grains coarsen. In addition, the time between passes means the time difference between the rolling of the end of the advancing direction of the previous pass, and the rolling of the trailing end of the advancing direction of this path. In addition, in the said manufacturing process, the reduction ratio in temperature exceeding 850 degreeC is not specifically limited, For example, it can set suitably in about 0 to 80% of range.

After hot rolling, it accelerates and cools. By accelerating cooling, coarsening of the martensite phase can be prevented, and the area percentage can be suppressed within a predetermined range. Although a cooling rate can be set suitably according to C amount etc., for example, it is recommended to cool the average speed to 5 degree-C / sec or more in the range of 700-500 degreeC after completion | finish of rolling.

The thick steel sheet of the present invention has a thickness of 3.0 mm or more according to the definition of JIS thick steel sheet, but preferably has a thickness that requires welding of heat input (especially superheat input) of 10 kJ / mm or more. The plate thickness for which 10 kJ / mm or more of heat input (particularly super heat input) is required is, for example, 20 mm or more, more preferably 40 mm or more, especially 60 mm or more. According to the present invention, in order to exhibit good HAZ toughness by welding a wide heat input amount ranging from a heat input amount of about 10 kJ / mm to a super input heat amount, even if the plate thickness is thick, welding can be performed without deteriorating the HAZ toughness. have.

EXAMPLE

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not inherently limited by the following example, It is carried out by changing suitably in the range which may be suitable for the meaning of the previous and the later. Of course, it is possible, and these are all included in the technical scope of this invention.

Experiment No. 1 to 60

Steels of the compositions shown in Tables 1 to 3 were dissolved by a common solvent method, and the molten steel was cooled (cooling rate from 1500 ° C to 1100 ° C: 0.1 to 2.0 ° C / sec) to obtain a slab (slab). Thickness = 270 mm). The slab was heated to a temperature of 1100 ° C. and then at a finish rolling temperature of 800 ° C. (for experiment Nos. 1 to 55 and 59 to 60) or a finish rolling temperature of less than 800 ° C. (for experiments No. 56 to 58). And hot rolling to thickness 60mm. Moreover, in this hot rolling, the time between each pass of the range of 850-800 degreeC was as showing in Tables 4-5. In the examples of Nos. 56 to 58, hot rolling is also performed in the range of less than 800 ° C, and the reduction ratio in this temperature range is 5% (No. 56), 20% (No. 57), or 30% (No. 58). After the end of hot rolling, it was immediately cooled to 200 ° C or lower. The cooling rate between the temperature 700-500 degreeC is as showing in Tables 4-5.

[Ti] / [N] and X values calculated from the chemical composition of the steel sheet, and values of the temperature range of the δ range calculated from Thermo-calc (indicated as "δ range" in the table) are shown in Tables 1 to 3 Shown in

In addition, the steel sheet manufactured as described above was examined for the former austenite particle diameter, the area percentage and aspect ratio of the phase martensite (MA), and the average particle diameter of Ti-based carbonitride. Further, as described below, tensile strength, transition temperature (vTrs) and HAZ toughness of the steel sheet were measured. These results are shown in Tables 4-5.

[The tensile strength]

Tensile strength was measured by extracting a JIS No. 4 test piece at a position of depth t / 4 (t = plate thickness) and performing a tensile test. A tensile strength of 440 MPa or more was regarded as pass.

[Transition Temperature (vTrs)]

The V notched standard test piece prescribed by JIS Z 2242 was taken at the position of depth t / 4 (t = plate thickness), the measurement temperature was changed, and the Charpy impact test (shock blade radius 2 mm) was performed, and the brittle fracture rate was 50%. The following temperature (vTrs) was investigated. The transition temperature (vTrs) was below -60 degreeC as the pass.

[HAZ Toughness]

1) Heat input 50 kJ / mm

SEGARC welding was performed at a heat input of 50 kJ / mm for a steel plate having a sheet thickness of 60 mm. A V notched standard test piece specified in JIS Z 2242 was taken from t / 2 part (t = plate thickness) shown in Fig. 1 (notch position is 0.5 mm HAZ side from the bond), and the Charpy impact test (impact) at -40 ° C. Blade radius 2 mm) was carried out to measure the absorbed energy (vE- ₄₀ ). The thing whose absorption energy is 200 J or more was made into the pass.

2) In case of heat input amount of 15 kJ / mm

It carried out similarly to experiment No.1-60, except having made the reduction ratio more than 850 degreeC high, and making the plate | board thickness of the steel plate obtained into 20 mm. That is, the rolling conditions and cooling conditions of 850 degrees C or less are the same as Experiment No. 1-60. The steel plate of 20 mm of plate | board thickness obtained in this way also gives and demonstrates the same number as the steel plate of 60 mm of said plate | board thicknesses. Electro-gas arc welding was performed with the heat input of 15 kJ / mm with respect to the steel plate of 20 mm of plate | board thickness. V notch standard test piece prescribed in JIS Z 2242 was taken from t / 2 part (t = plate thickness) (notch position is 0.5 mm HAZ side from bond), and Charpy impact test (shock blade radius 2mm at -40 degreeC) ), And the absorbed energy (vE _-40 ) was measured. The thing whose absorption energy is 200 J or more was made into the pass.

Table 1

[Table 2]

[Table 3]

Table 4

Table 5

Further, based on the results of Experiments Nos. 1 to 36 satisfying the component range of the present invention, the HAZ toughness (vE _-40 ) at X value, average particle diameter of Ti-based carbonitride, and heat input amount of 50 kJ / mm, The relationship of HAZ toughness (vE- ₄₀ ) at the heat input amount of 15 kJ / mm was summarized. The results are shown in FIGS. 2 to 4.

As apparent from Figs. 2 to 4, by increasing the X value, the average particle diameter of the Ti-based carbonitride can be reduced, and the HAZ toughness (vE _-40 ) at a heat input amount of 50 kJ / mm can be improved. In addition, the HAZ toughness (vE _-40 ) at a heat input amount of 15 kJ / mm can also be improved. Further, as shown in Tables No. 1 to 35 of Table 4, the transition temperature can also be lowered by making the old austenite grain size finer and making the martensite MA smaller and rounder.

On the other hand, Experiment No. 36, 51, and 52 are examples where X value is too small, and HAZ toughness deteriorates. No. 50 had too large an X value and excessively increased island martensite (MA), so that the transition temperature was high and the HAZ toughness also deteriorated. Nos. 37 to 49 had deteriorated HAZ toughness because of inadequate component range or Ti / N ratio.

Nos. 53 to 55 had a slow cooling rate after hot rolling, and increased transition temperature because the island martensite (MA) was excessively increased. As for Nos. 56-58, as a result of reducing the reduction ratio of 850-800 degreeC, the reduction amount under less than 800 degreeC became large, phase martensite MA extended | stretched, and transition temperature became high. Nos. 59 and 60 had an inadequate interpass time when rolling between 850 and 800 ° C, and the transition temperature was high because the old austenite grains were coarsened.

The tensile strength class of the thick steel sheet of the present invention is, for example, 440 MPa or more, preferably 490 MPa or more, more preferably 540 MPa or more, and in the most preferred case, a steel plate of 590 MPa or more can also be provided. . The thick steel sheet of the present invention can be applied, for example, to welded structures such as ships and offshore structures, and particularly high tensile steel sheets excellent in tensile strength are suitable for manufacturing large container ships and the like.

1 is a schematic diagram showing a sampling position of a test piece for measuring HAZ toughness.

2 is a graph summarizing the relationship between the X value and the average particle diameter of Ti-based carbonitride based on the results of Experiments Nos. 1 to 36. FIG.

Fig. 3 is a graph summarizing the relationship between the X value and the HAZ toughness (vE- ₄₀ ) at a heat input amount of 15 kJ / mm based on the results of Experiments Nos. 1 to 36;

4 is a graph summarizing the relationship between the X value and the HAZ toughness (vE- ₄₀ ) at a heat input amount of 50 kJ / mm based on the results of Experiments Nos. 1 to 36. FIG.

Claims (6)

C: 0.030 to 0.10% (meaning of mass%, the same below), Si: 1.0% or less (0% not included), Mn: 0.8 to 2.0%, P: 0.03% or less (0% not included) ), S: 0.01% or less (not including 0%), Al: 0.01% to 0.10%, Nb: 0.005 to 0.035%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, and N: 0.0050 to 0.01% Containing, Cu: 2.0% or less (including 0%), Ni: 2.0% or less (including 0%), Cr: 1% or less (including 0%), Mo: 0.5% or less (including 0%) And V: 0.1% or less (including 0%), The remainder is a thick steel plate made of Fe and unavoidable impurities, The average equivalent circle diameter of the old austenite grain is 120 µm or less, the area percentage of the island martensite (MA) is 3% or less, and the number average value of the aspect ratio (long diameter / short diameter) of the island martensite (MA) is 3 or less ego, Furthermore, the thick steel sheet excellent in HAZ toughness and base material toughness characterized by satisfying the following formulas (1) and (2). [Equation 1] 1.5 ≤ [Ti] / [N] ≤ 4 [Equation 2] 40 ≤ X value ≤ 160 X value = 500 [C] + 32 [Si] + 8 [Mn]-9 [Nb] + 14 [Cu] + 17 [Ni]-5 [Cr]-25 [Mo]-34 [V] [Ti], [N], [C], [Si], [Mn], [Nb], [Cu], [Ni], [Cr], [Mo], and [V] in the formula Content (mass%) of each element] The thick steel sheet according to claim 1, wherein the temperature range in the δ range is 40 ° C or less. The thick steel sheet according to claim 1 or 2, wherein the Ti-based carbonitride has an average particle diameter of 40 nm or less at a position of depth t / 4 (t = plate thickness). The method according to claim 1 or 2, further comprising Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), and REM: 0.01% or less (not including 0%). Thick steel sheet containing at least one selected from The thick steel sheet according to claim 1 or 2, further comprising at least one selected from Zr: 0.1% or less (not containing 0%) and Hf: 0.05% or less (not containing 0%). The thick steel sheet according to claim 1 or 2, further comprising at least one selected from Co: 2.5% or less (does not contain 0%) and W: 2.5% or less (does not contain 0%).

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