JP2009179844A - High tensile strength thick steel plate having excellent toughness in weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength thick steel plate having excellent toughness in the weld heat affected zone in high heat input welding. <P>SOLUTION: The high tensile strength thick steel plate has a composition comprising, by mass, 0.02 to 0.12% C, 0 to 0.40% Si, 1.0 to 2.0% Mn, 0 to 0.03% P, 0 to 0.015% S, 0 to 0.050% Al, 0.005 to 0.100% Ti, 0.0002 to 0.0500% REM and/or 0.0003 to 0.0100% Ca, 0.0001 to 0.0500% Zr, 0.0020 to 0.0300% N and 0.0005 to 0.0100% O, and the balance Fe with inevitable impurities, in which the number of oxides present in the steel and having an equivalent circle diameter of <2 μm is ≥500 pieces/mm<SP>2</SP>, also, the number of oxides present in the steel and having an equivalent circle diameter of >5 μm is ≤5 pieces/mm<SP>2</SP>, and X value defined by formula (1): X=100×[insol Mn]/([insol REM]+[insol Ca]+0.05) from the concentrations of Mn, REM and Ca forming a compound in the steel is <10.0. In the formula, [insol Mn], [insol REM] and [insol Ca] denote the concentrations (mass%) of Mn, REM and Ca forming a compound in the steel, respectively. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is used as a structural material in the fields of shipbuilding, construction, and the like, and has high toughness for welding excellent in toughness of a weld heat affected zone (hereinafter referred to as “HAZ”) for application of high heat input welding. It relates to steel plates.

  In general, steel materials used as structural materials in fields such as shipbuilding and construction are often joined by welding, and it is essential that they have excellent HAZ toughness in addition to base metal toughness.

  In recent years, with the increase in size of welded structures in the building and shipbuilding fields, the application range of thick steel plates with a thickness of 50 mm or more is expanding, and high heat input welding is desired from the viewpoint of reducing welding construction costs. However, in general, HAZ of thick steel plates is liable to deteriorate toughness, and the tendency becomes more remarkable particularly in high heat input welding. That is, in high heat input welding, since HAZ is gradually cooled after being held in a high temperature austenite region by heating during welding, the coarsening of the structure such as austenite grain growth and generation of coarse grain boundary ferrite from the austenite grain boundary. As a result, toughness deterioration associated with this is a major issue.

  In response to this problem, as a technique for ensuring the toughness of HAZ (hereinafter referred to as HAZ toughness), γ grain coarsening suppression technology using inclusions such as oxides, sulfides, or nitrides can be roughly divided. , And intragranular α-transformation promotion techniques have been proposed. The former is a technique for suppressing the austenite grain coarsening during high-temperature heating by pinning austenite grain growth by inclusion particles dispersed in the steel, and the latter is a technique for obtaining a fine structure. It is a technique for obtaining a microstructure by utilizing the α transformation as a starting point and promoting intragranular α transformation in the slow cooling process after the end of welding.

  Conventionally, TiN has been mainly used as an inclusion effective for suppressing γ grain coarsening and promoting intragranular α transformation. For example, Japanese Laid-Open Patent Publication No. 2001-20031 (Patent Document 1) proposes a HAZ toughness improving technique using Ti-REM-Ca-Al-based oxides with appropriately controlled compositions and TiN. Japanese Patent Application Laid-Open No. 2003-166017 (Patent Document 2) discloses a technique using suppression of γ grain coarsening by TiN and promotion of intragranular α transformation by Mn sulfide. Furthermore, Japanese Patent Laid-Open No. 2003-321728 (Patent Document 3) proposes a technique for obtaining high HAZ toughness by combining the suppression of γ grain coarsening by TiN containing Mg oxide and the promotion of intragranular α transformation by MnCaS. ing.

  However, in contrast to the recent trend of increasing welding heat input, TiN is prone to disappearance and coarsening during welding, making it difficult to ensure stable HAZ toughness. On the other hand, technologies using oxides, sulfides, or oxysulfides that are stable at high temperatures have been proposed as inclusions effective in suppressing γ grain coarsening and promoting intragranular α transformation, and many studies have been made. .

For example, Japanese Patent Application Laid-Open No. 2005-206910 (Patent Document 4) discloses a technique for suppressing γ grain coarsening with REM and Mn-containing oxysulfides and obtaining high HAZ toughness. Japanese Patent Laid-Open No. 2003-286540 (Patent Document 5) proposes a technique for finely dispersing Mn oxysulfide by appropriately controlling REM and suppressing γ grain coarsening. Japanese Unexamined Patent Application Publication No. 2007-100133 (Patent Document 6) proposes a technique for obtaining high HAZ toughness by suppressing γ grain coarsening using an oxide containing REM and Zr. Japanese Patent Publication No. 4-54734 (Patent Document 7) proposes a technique in which BN is precipitated in an REM or Ca oxide or sulfide, and is used as a starting point of intragranular α transformation.
Japanese Patent Laid-Open No. 2001-20031 Japanese Patent Laid-Open No. 2003-166017 JP 2003-321728 A JP 2005-206910 A JP 2003-286540 A Japanese Patent Laid-Open No. 2007-1001000 Japanese Examined Patent Publication No. 4-54734

  However, the increase in the maximum heating temperature and the prolonged high-temperature holding time accompanying the recent increase in welding heat input promotes the coarsening of the structure and at the same time leads to Mn sulfide dissolution in the HAZ during welding and reprecipitation during the subsequent cooling process. The Mn sulfide fine particles that increase the HAZ hardness by precipitation strengthening cause a decrease in HAZ toughness. Therefore, the effect of improving the HAZ toughness is inevitably limited only by the conventional structure refinement technique, and a means for obtaining a better HAZ toughness is desired.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-tensile steel plate for welding having excellent HAZ toughness particularly in high heat input welding.

  In order to achieve the above-mentioned problems, the present inventors conducted experiments and examinations on means for obtaining excellent HAZ toughness by simultaneously achieving microstructure refinement by inhibiting γ grain coarsening and fine Mn sulfide reprecipitation inhibition. went. As a result, by appropriately controlling the casting process, it is possible to disperse an oxide having an equivalent circle diameter smaller than 2 μm at a high density, which is effective in suppressing γ grain coarsening, and to form a compound in steel. Based on the concentrations of REM and Ca, the parameter indicating the amount of Mn sulfide present in the steel (X value to be described later) is controlled to be less than a predetermined value to suppress the reprecipitation of fine Mn sulfide. I found it. The present invention has been completed based on such knowledge.

That is, the high-tensile steel plate for welding of the present invention is in mass% (hereinafter, “mass%” is simply described as “%”),
C: 0.02 to 0.12%,
Si: 0.40% or less (including 0%),
Mn: 1.0-2.0%,
P: 0.03% or less (including 0%),
S: 0.015% or less (including 0%),
Al: 0.050% or less (including 0%),
Ti: 0.005 to 0.100%,
REM: 0.0002 to 0.0500% and / or Ca: 0.0003 to 0.0100%,
Zr: 0.0001 to 0.0500%
N: 0.0020 to 0.0300%,
O: 0.0005 to 0.0100%
The balance is composed of Fe and inevitable impurities. Organizationally, the number of oxides having an equivalent circle diameter of less than 2 μm existing in steel is 500 / mm ² or more and the number of oxides having an equivalent circle diameter of more than 5 μm is 5 / mm ² or less. The X value defined by the formula (1) is less than 10.0 based on the concentrations of Mn, REM, and Ca forming the compound.

Further, the above basic components include Group A (Ni: 0.05-1.50%, Cu: 0.05-1.50%, Cr: 0.05-1.50%, Mo: 0.05-1. 50%), Group B (Nb: 0.002-0.10%, V: 0.002-0.10%), B: 0.0010-0.0050%, one or more elements added Thus, the following chemical compositions (1) to (3) can be obtained.
(1) Basic component + 1 or more from Group A
(2) Basic component or one or more of component (1) above + Group B
(3) Basic component, component (B) above (1) or component (B) above + B

  According to the high-strength thick steel plate for welding of the present invention, with a predetermined steel composition, a small-diameter oxide having an equivalent circle diameter of less than 2 μm is used to suppress the coarsening of γ grains and to refine the structure and to start the brittle fracture As a compound composition that can suppress the formation of large-diameter oxides with an equivalent circle diameter of more than 5 μm and further suppress the reprecipitation of fine Mn sulfide, And excellent HAZ toughness.

First, the high-strength steel sheet for welding according to an embodiment of the present invention will be described in detail below from the structural conditions.
In general, to suppress γ grain coarsening, it is necessary to disperse fine inclusion particles at high density. As the particle size increases, the number density of inclusion particles decreases, and γ grain coarsening is suppressed. become unable. Accordingly, the present inventors have determined experimentally the inclusion particle size and number density at which a sufficient γ grain coarsening suppression effect can be obtained, and disperse 500 oxides / mm ² or more of oxides having an equivalent circle diameter smaller than 2 μm. Thus, it was found that γ grain coarsening is suppressed. When the circle equivalent diameter of 2μm smaller oxide is less than 500 / mm ^2, necessary γ grains coarsening can not be obtained. Note that the number of oxides having an equivalent circle diameter of less than 2 μm is preferably 800 / mm ² or more.

In addition, as a means of suppressing fine Mn sulfide reprecipitation in HAZ, the present inventors have focused on REM sulfide and / or Ca sulfide, which are stable at higher temperatures than Mn sulfide, and have added compounds in steel. By controlling the X value defined as the following formula (1) to less than 10.0 from the concentration of Mn, REM, and Ca to be formed, the main component of sulfide becomes Ca sulfide and / or REM sulfide, and HAZ It has been found that the reprecipitation of fine Mn sulfides in is suppressed.
X = 100 × [insol Mn] / ([insol REM] + [insol Ca] +0.05) (1)
However, [insol Mn], [insol REM], and [insol Ca] indicate the concentrations (%) of Mn, REM, and Ca that form compounds in the steel, respectively.

  The technical meaning of the X value is as follows. The X value is a parameter expressing the amount of Mn sulfide present in the steel. For Mn sulfide, the number or size of Mn sulfide particles is generally evaluated by structural observation using an optical microscope, a scanning electron microscope, or the like. However, in the component system of the present invention, Mn sulfide is rarely present alone, and most of the Mn sulfide is present as composite particles with REM sulfide, Ca sulfide, and the like. It is difficult to find. Therefore, in this invention, the method of indirectly evaluating the amount of Mn sulfide by the X value was adopted. That is, when the X value is less than 10.0, the amount of REM and Ca existing as a compound is sufficiently secured with respect to the amount of Mn present as a compound, so that the main component of sulfide is REM sulfide that is more stable at high temperatures. , Ca sulfide, and reprecipitation of fine Mn sulfide in HAZ is suppressed. When the X value is 10.0 or more, the main component of sulfide in the steel is Mn sulfide, and reprecipitation of fine Mn sulfide is not sufficiently suppressed. The X value is preferably less than 8.0.

In order to obtain good HAZ toughness, in addition to the above-mentioned fine oxide dispersion and fine Mn sulfide reprecipitation suppression, the number of oxides having an equivalent circle diameter larger than 5 μm in steel is 5 pieces / mm ² or less. Need to control. When the number of oxides having an equivalent circle diameter larger than 5 μm exceeds 5 / mm ² , these oxides act as starting points for brittle fracture, and the toughness deteriorates. The number of oxides having an equivalent circle diameter larger than 5 μm is preferably 3 mm ² or less.

In order to obtain the above-mentioned oxide distribution and X value, the amount of dissolved oxygen, REM and / or Ca and Zr alloy elements added in the casting process, and the time from the completion of alloy element addition to the start of casting are controlled as follows. There is a need to. That is, at the time of casting, Mn, Si, Al, etc. are added prior to Ti addition, and the dissolved oxygen amount before Ti addition is controlled to 0.0020 to 0.0100%, and the dissolved oxygen amount and sulfur amount before Ti addition. Based on the above, after determining the amount of REM and / or Ca and Zr added so that the Z value obtained from the following formula (2) is 0.58 or more, after adding Ti, REM and / or Ca, In addition, Zr is added, and further, the time t (min) from the completion of the addition of these alloy elements to the start of casting is maintained within the range defined by the following formula (3), and solidification proceeds at the time of casting: 1450-1500 The cooling time at 0 ° C. is controlled to 60 to 300 s.
Z = (3.5 × ([REM] + [Ca]) − 0.7 × [O] + 2.6 × [Zr] +0.3) / ([S] +0.5) (2)
3 + 200 × [O] × [S] / ([REM] + [Ca]) <t <60 …… (3)
In addition, [O] and [S] are each expressed in%, and are dissolved O amount and dissolved S amount before addition of Ti, and [REM], [Ca], and [Zr] are each expressed in%. This is the amount of REM, Ca, Zr added.

  At the time of casting, if the amount of dissolved oxygen before addition of Ti is less than 0.0020%, a sufficient amount of oxide having an equivalent circle diameter of less than 2 μm cannot be secured. Further, when the dissolved oxygen amount is more than 0.0100%, or when REM and / or Ca and Zr are added prior to Ti, the amount of oxide having an equivalent circle diameter larger than 5 μm is increased or the equivalent circle diameter is increased. A sufficient amount of oxide smaller than 2 μm cannot be obtained.

  The Z value is a value that takes into account the amount of REM, Ca, and S that contribute to the formation of REM sulfide and / or Ca sulfide. If the Z value is less than 0.58, the S value relative to REM and / or Ca Since the ratio becomes too high, the amount of Mn sulfide produced at a lower temperature or the amount of solute S is increased, and the amount of reprecipitation of fine Mn sulfide in HAZ increases. Each coefficient in the formula was experimentally determined with respect to the definition formula of the Z value and t (min).

  Further, if the time t (min) from the completion of addition of the alloy element to the start of casting deviates from the range of the above formula (3), a sufficient HAZ toughness improving effect cannot be obtained. That is, when the value of t is 3 + 200 × [O] × [S] / ([REM] + [Ca]) or less, the formation of REM sulfide and / or Ca sulfide in the molten steel does not proceed sufficiently, Reprecipitation of fine Mn sulfide is not sufficiently suppressed. On the other hand, when the value of t is 60 or more, oxides having an equivalent circle diameter larger than 5 μm increase due to coalescence / growth of oxides, which causes a reduction in HAZ toughness.

  Further, if the cooling time for passing the temperature range of 1450 to 1500 ° C. is shorter than 60 s after casting the molten metal whose components are adjusted into the mold and before solidification, REM sulfide and / or Ca sulfide is used as a secondary inclusion. Since sufficient time for the formation of the product is not ensured, the amount of Mn sulfide produced at a lower temperature or the amount of solid solution S is increased, and the amount of reprecipitation of fine Mn sulfide in HAZ increases. On the other hand, when the cooling time exceeds 300 s, formation of an oxide having an equivalent circle diameter larger than 5 μm is promoted in the alloy element concentration region due to solidification segregation, resulting in a decrease in HAZ toughness.

Next, basic components of the chemical composition of the high-tensile thick steel plate for welding according to the embodiment will be described.
C: 0.02-0.12%
C is an element essential for securing the strength, and if the content is less than 0.02%, the required strength cannot be obtained, so the lower limit was made 0.02%. On the other hand, if the content exceeds 0.12%, the HAZ toughness decreases due to an increase in the hard MA structure, so the upper limit was made 0.12%. The preferred lower limit is 0.04%, and the preferred upper limit is 0.10%.

Si: 0.40% or less (including 0%)
Si is an element that secures strength by solid solution strengthening, and if the content exceeds 0.40%, a hard MA structure (mixed structure of martensite and residual austenite) increases and HAZ toughness decreases. The upper limit was made 0.40%. Note that the content is preferably 0.35% or less (including 0%).

Mn: 1.0-2.0%
Mn is an element effective for securing the strength, and if it is less than 1.0%, the required strength cannot be obtained, so the lower limit was made 1.0%. Further, if the content exceeds 2.0%, the HAZ strength is excessively increased and the HAZ toughness is lowered, so the upper limit was made 2.0%. The preferred lower limit is 1.4% and the preferred upper limit is 1.8%.

P: 0.03% or less (including 0%)
P is an impurity element that causes grain boundary fracture due to grain boundary segregation. If the content exceeds 0.03%, the HAZ toughness is reduced, so the upper limit was made 0.03%. Note that the content is preferably 0.02% or less (including 0%).

S: 0.015% or less (including 0%)
S is an element that is present as Mn sulfide and causes HAZ toughness reduction due to reprecipitation of fine Mn sulfide in HAZ. When the content exceeds 0.015%, reprecipitation of Mn sulfide is suppressed. Therefore, the upper limit was made 0.015%. Note that the content is preferably 0.012% or less (including 0%).

Al: 0.050% or less (including 0%)
Al is an element used for deoxidation at the time of casting. When the content exceeds 0.050%, a coarse oxide is formed and the HAZ toughness is reduced, so the upper limit was made 0.050%. In addition, Preferably it is 0.040% or less.

Ti: 0.005 to 0.100%
Ti is an element that contributes to the formation of fine oxides by adding prior to REM and Zr. If the content is less than 0.005%, a sufficient effect cannot be obtained, so the lower limit is set to 0.005. %. Further, if the content exceeds 0.100%, the HAZ toughness is reduced due to the coarsening of the oxide, so the upper limit was made 0.100%. A preferred lower limit is 0.010%, a preferred upper limit is 0.080%, a more preferred upper limit is 0.060%, and a more preferred upper limit is 0.050%.

REM: 0.0002-0.0500% and / or Ca: 0.0003-0.0100%
REM (rare earth element) and Ca form REM sulfide and Ca sulfide, respectively, thereby reducing the amount of Mn sulfide and suppressing the reduction in HAZ toughness due to reprecipitation of fine Mn sulfide. In order to obtain these effects sufficiently, it is necessary to contain 0.0002% or more in REM and 0.0003% or more in Ca. Preferably, the REM content is 0.0005% or more, and the Ca content is 0.0010% or more. However, if the content of these elements is excessive, the HAZ toughness is lowered due to the formation of coarse oxides, so it is necessary to make 0.0500% or less in REM and 0.0100% or less in Ca. Preferably, the REM is 0.0400% or less, and the Ca is 0.0080% or less.

Zr: 0.0001 to 0.0500%
Zr is an element that contributes to the formation of fine oxides with an equivalent circle diameter of less than 2 μm when added after the addition of Ti during casting. If the content is less than 0.0001%, the effect is sufficient. Therefore, the lower limit was made 0.0001%. Further, if the content is more than 0.0500%, coarse oxides or fine carbides that cause precipitation strengthening are formed and the toughness is reduced, so the upper limit was made 0.0500%. A preferred lower limit is 0.0005%, and a preferred upper limit is 0.0400%.

N: 0.0020 to 0.0300%
N is an element that forms Ti nitride to improve toughness, and if the content is less than 0.0020%, a sufficient effect cannot be obtained, so the lower limit was made 0.0020%. Moreover, when content exceeds 0.0300%, since the toughness fall by strain aging will be caused as solid solution N, the upper limit was made 0.0300%. The preferred lower limit is 0.0030%, the preferred upper limit is 0.0250%, the more preferred upper limit is 0.0200%, and the still more preferred upper limit is 0.0150%.

O: 0.0005 to 0.0100%
O is an essential element for the production of fine oxides. If the content is lower than 0.0005%, a sufficient amount of oxide cannot be obtained, so the lower limit was made 0.0005%. Further, if the content exceeds 0.0100%, the HAZ toughness is reduced due to the coarsening of the oxide, so the upper limit was made 0.0100%. A preferred lower limit is 0.0010%, and a preferred upper limit is 0.0080%.

In order to further improve the mechanical properties of the steel sheet with respect to the basic component, the basic component includes group A (Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0). 0.05 to 1.50%, Mo: 0.05 to 1.50%), Group B (Nb: 0.002 to 0.10%, V: 0.002 to 0.10%), B: 0.0. One or more of 0010 to 0.0050% can be added to obtain the following compositions (1) to (3) (the balance being Fe and inevitable impurities).
(1) Basic component + 1 or more from Group A
(2) Basic component or one or more of component (1) above + Group B
(3) Basic component, component (B) above in either (1) or (2)

  Ni, Cu, Cr, and Mo are all effective elements for increasing the strength of steel materials. If the content is less than 0.05%, the effect cannot be sufficiently obtained. 05%. In addition, when the content exceeds 1.50%, the strength is excessively increased and the toughness is reduced. Therefore, the upper limit is set to 1.50%. The preferred lower limit is 0.10%, and the preferred upper limit is 1.20%.

  Nb and V are elements that suppress austenite grain coarsening by precipitating as carbonitrides, and if the content is less than 0.002%, the effect cannot be sufficiently obtained. The lower limit was made 0.002%. In addition, when the content exceeds 0.10%, the coarse carbonitride causes a reduction in toughness, so the upper limit was made 0.10%. The preferred lower limit is 0.005% and the preferred upper limit is 0.08%.

  B is an element that improves toughness by suppressing the formation of intergranular ferrite. If the content is less than 0.0010%, the effect cannot be sufficiently obtained, so the lower limit is made 0.0010%. . On the other hand, if the content is more than 0.0050%, BN precipitates at the austenite grain boundary and causes a decrease in toughness, so the upper limit was made 0.0050%. A preferred lower limit is 0.0015%, and a preferred upper limit is 0.0040%.

  Next, the manufacturing method of the said high-tensile steel plate for welding is demonstrated. The feature of the method for producing the same steel plate is the casting process, which has already been described, and hereinafter, the processing for the steel ingot after casting will be briefly described. The ingot produced by satisfying the above casting process, component range, etc., was rolled according to a normal hot rolling procedure, with a rolling start temperature of about 1200 to 900 ° C. and a rolling end temperature of about 950 to 750 ° C. Then, a thick steel plate is obtained by cooling to the cooling stop temperature between room temperature and about 500 degreeC. A temper treatment may be further performed after the cooling. In addition, although it does not restrict | limit especially about the plate | board thickness of the steel plate manufactured, About 50-120mm is calculated | required as a steel plate for welding, and this invention obtains the outstanding HAZ toughness even if it is this plate | board thickness. Can do.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.

Using a vacuum melting furnace (150 kg), adjust the amount of silicon added to change the amount of dissolved oxygen before adding Ti, and determine the amount of REM and / or Ca and Zr added while considering the Z value. , Except for some (steel No. 26 in Table 1), after adding Ti, add REM and / or Ca, Zr, time t (min) from completion of addition of alloy element to start of casting, and After casting the component-adjusted molten metal in the mold, the steel is melted by changing the cooling time t1 (s) when passing through the temperature range of 1450 to 1500 ° C. until solidification, and the resulting ingot is obtained. Hot rolling was performed at a rolling start temperature of about 950 ° C. and a final rolling temperature of about 880 ° C. to produce a thick steel plate having a thickness of 80 mm. The compositions are shown in Tables 1 and 2, and the amount of dissolved oxygen before adding Ti [O] (%), the Z value, and the allowable t (min) expressed by the following equation (4) based on the equation (3) Table 3 shows the lower limit values Y, t (min), and t1 (s).
Y = 3 + 200 × [O] × [S] / ([REM] + [Ca]) (4)
[O] and [S] are respectively the dissolved O amount (%) and the dissolved S amount (%) before Ti addition, and [REM] and [Ca] are respectively the addition amount (%) of REM and Ca. is there.

  At the position t (plate thickness) / 4 of each thick steel plate obtained, the concentrations of Mn, REM, and Ca forming the compound were measured, and the X value was calculated. About the density | concentration of Mn which forms a compound, it measured by the iodine methanol method. Further, the concentrations of REM and Ca forming the compound were measured by an electrolytic extraction method using a solution containing 2 cc of triethanolamine and 1 g of tetramethylammonium chloride in 100 cc of methanol as an electrolytic solution. A filter having a pore size of 0.1 μm was used for filtering the residue. The obtained X values are shown in Table 3.

  In addition, a test piece was cut out from each of the obtained thick steel plates at t (plate thickness) / 4 position, and a cross section parallel to the rolling direction and the plate thickness direction (cross section perpendicular to the rolling surface and along the rolling direction) was field emission type. Observed using a scanning electron microscope (device name: SUPRA 35, manufactured by Carl Zeiss) (hereinafter referred to as FE-SEM), an oxide having an equivalent circle diameter of less than 2 μm and an oxide of more than 5 μm The number density was measured. The measurement method is as follows.

First, the observation magnification of FE-SEM was set to 5000 times, 20 visual fields having an area of 0.0024 mm ² were randomly selected, and images of each visual field were taken. At the same time, the central part of each inclusion particle having a maximum diameter of 2 μm or less included in each field of view was measured with EDS attached to the FE-SEM, and the inclusion particle containing oxygen as a constituent element was determined to be an oxide. Inclusion particles having a maximum diameter of 0.2 μm or less were excluded from the measurement target because of the low reliability of EDS measurement. After that, the obtained image was subjected to image analysis using an image processing software (software name: Image-Pro Plus, manufactured by Media Cybernetic). Among these oxides, the number density N1 (number of ones having an equivalent circle diameter of less than 2 μm) / Mm ² ) was calculated. The value of N1 is shown in Table 3. Similarly, the number density NL (pieces / mm ² ) of oxides having an equivalent circle diameter larger than 5 μm, calculated from 20 fields of view having an area of 0.06 mm ² , with the observation magnification of FE-SEM set to 1000 times. Table 3 shows the values.

Furthermore, from the t (plate thickness) / 4 position of each thick steel plate obtained, a Charpy test piece (V notch) was sampled in parallel to the rolling direction, and the HAZ thermal cycle during large heat input welding was simulated, The toughness when subjected to reproducible HAZ thermal cycling was evaluated. The reproduced HAZ thermal cycle is a test piece heated to 1400 ° C. and held for 60 s, and then cooled in a temperature range of 800 to 500 ° C. over 500 s. The Charpy impact test conforms to JIS Z 2242, and the minimum value vE- ₄₀ (min) of the measured values of impact absorption energy vE- ₄₀ (J) at -40 ° C for three test pieces exceeds 100J. Was evaluated as having excellent HAZ toughness. Table 3 shows the value of vE _-40 (min).

As apparent from Tables 1 to 3, Invention Examples Nos. 1 to 25 appropriately controlled the composition of the thick steel plate and the casting process, so that the oxide was dispersed in an appropriate form, and the X value was 10. Succeeded to make it less than 0. For this reason, microstructure refinement and fine Mn sulfide reprecipitation suppression can be achieved, and a high value was obtained in HAZ toughness (vE _-40 (min)). On the other hand, in Comparative Examples Nos. 26 to 47, the order of addition of Ti, REM, and Zr, the amount of dissolved oxygen [O] (%) before Ti addition, and the time t ( min), and after casting, the cooling time t1 (s) when passing through the temperature range of 1450 to 1500 ° C., or the composition of the steel deviates from an appropriate range, and the prescribed oxide form cannot be obtained. Or because the X value exceeded 10.0, or perhaps because of increased coarse inclusions, increased impurities, excessive strengthening, grain boundary segregation of solid solution elements, etc. It has dropped considerably.

Claims (4)

% By mass
C: 0.02 to 0.12%,
Si: 0.40% or less (including 0%),
Mn: 1.0-2.0%,
P: 0.03% or less (including 0%),
S: 0.015% or less (including 0%),
Al: 0.050% or less (including 0%),
Ti: 0.005 to 0.100%,
REM: 0.0002 to 0.0500% and / or Ca: 0.0003 to 0.0100%,
Zr: 0.0001 to 0.0500%
N: 0.0020 to 0.0300%,
O: 0.0005 to 0.0100%
In which the balance is Fe and inevitable impurities, and the equivalent circle diameter in the steel is 500 / mm ² or more and the equivalent circle diameter is 5 μm or more. Large heat input, in which the number of larger oxides is 5 / mm ² or less, and the X value defined by the formula (1) is less than 10.0 based on the concentration of Mn, REM, and Ca forming a compound in steel High strength thick steel plate for welding with excellent toughness of heat affected zone during welding.
X = 100 × [insol Mn] / ([insol REM] + [insol Ca] +0.05) (1)
However, [insol Mn], [insol REM], and [insol Ca] indicate the concentrations (mass%) of Mn, REM, and Ca that form compounds in the steel, respectively. Furthermore, in mass%,
Ni: 0.05-1.50%,
Cu: 0.05 to 1.50%,
Cr: 0.05 to 1.50%,
Mo: 0.05 to 1.50%,
The high-tensile thick steel plate for welding according to claim 1, comprising one or more of them. Furthermore, Nb by mass%: 0.002 to 0.10%,
V: 0.002-0.10%
The high-tensile steel plate for welding according to claim 1, comprising one or more of them. Furthermore, B by mass%: 0.0010 to 0.0050%
The high-tensile thick steel plate for welding according to any one of claims 1 to 3, comprising: