KR20090083844A - High-tensile strength thick steel plate with excellent toughness of weld heat affected zone - Google Patents

Abstract

A high tensile thick-sheet iron with a superior toughness of the weld heat-affected zone is provided to plan the microstructure by suppressing gamma-particle coarsening using small-diameter oxide less than 2mum. A high tensile thick-sheet iron with a superior toughness of the weld heat-affected zone contains C: less than 0.12%, Si: less than 0.02 (0% is included), Mn: 2.0%, P: less than 0.03% (0% is included), S: less than 0.015% (0% is included) Al: less than 0.050% (0% is included) Ti: 0.005 to 0.100%, REM: 0.0002 to 0.0500%, Ca: 0.0003 to 0.0100%, Zr: 0.0001 to 0.0500%, N: 0.0020 to 0.0300%, O: 0.0005 to 0.0100%.

Description

High-strength thick steel plate with excellent toughness of weld heat affected zone {HIGH-TENSILE STRENGTH THICK STEEL PLATE WITH EXCELLENT TOUGHNESS OF WELD HEAT AFFECTED ZONE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high tension thick steel sheet for welding, which is used as a structural material in fields of shipbuilding, construction, and the like, and has excellent toughness of a weld heat influencer (hereinafter referred to as "HAZ") for application of high heat input welding.

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

In recent years, with the increase in the size of the welded structure in the field of construction and shipbuilding, the application range of the thick steel plate of 50 mm or more of plate | board thickness continues to expand, and high heat input welding is calculated | required from a viewpoint of the welding construction cost reduction. In general, however, the HAZ of the thick steel sheet tends to cause toughness deterioration, and especially in the high heat input welding, the tendency becomes more remarkable. That is, in the high heat input welding, the HAZ is slowly cooled after being maintained in the high temperature austenite region by heating at the time of welding, so that tissue coarsening such as austenite grain growth and coarse grain boundary ferrite generation from the austenite grain boundary is likely to occur. In other words, the deterioration of toughness is a major problem.

To this end, as a technique for securing the toughness of HAZ (hereinafter referred to as HAZ toughness), there are largely divided γ grain coarsening suppression techniques using an inclusion such as oxide, sulfide, or nitride, and α transformation promoting technology in particles. Proposed. The former is a technique of obtaining fine structure by suppressing coarsening of austenite grains at high temperature by pinning austenite grain growth by inclusion particles dispersed in steel, and using the inclusion as a starting point of α transformation in particles. It is a technique of obtaining a microstructure by promoting the intra-particle alpha transformation in the slow cooling process after completion | finish.

Conventionally, TiN has been mainly used as an effective inclusion for suppressing γ grain coarsening and promoting α transformation in particles. For example, Japanese Patent Application Laid-Open No. 2001-20031 (Patent Document 1) discloses a technique for improving HAZ toughness using Ti-REM-Ca-Al-based oxide and TiN in which the composition is appropriately controlled. In addition, Japanese Patent Application Laid-Open No. 2003-166017 (Patent Document 2) describes a technique using inhibition of γ grain coarsening by TiN and acceleration of α transformation in particles by Mn sulfide. Further, Japanese Patent Application Laid-Open No. 2003-321728 (Patent Document 3) discloses a technique for obtaining high HAZ toughness by combining inhibition of γ grain coarsening by TiN containing Mg oxide and acceleration of α transformation in particles by MnCaS. It is proposed.

However, with respect to the recent tendency of heat input of welding, since TiN easily loses and coarsens during welding, it is difficult to secure stable HAZ toughness. On the other hand, techniques using oxides, sulfides, or oxysulfides stable at high temperatures have been proposed as effective inclusions for suppressing γ grain coarsening and promoting α transformation in particles, and many studies have been made.

For example, Japanese Patent Application Laid-Open No. 2005-206910 (Patent Document 4) describes a technique of suppressing gamma grain coarsening by REM and Mn-containing sulphides to obtain high HAZ toughness. In addition, Japanese Patent Application Laid-Open No. 2003-286540 (Patent Document 5) discloses a technique for finely dispersing Mn acid sulfide by appropriately controlling REM to suppress γ grain coarsening. In addition, Japanese Patent Application Laid-Open No. 2007-100213 (Patent Document 6) proposes a technique for obtaining high HAZ toughness by suppressing gamma grain coarsening using oxides containing REM and Zr. In addition, Japanese Patent Application Laid-Open No. 4-54734 (Patent Document 7) proposes a technique in which BN is precipitated in oxides or sulfides of REM or Ca to be the starting point of α transformation in particles.

<Patent Document 1> Japanese Patent Application Laid-Open No. 2001-20031

Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-166017

Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-321728

Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-206910

Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-286540

Patent Document 6: Japanese Unexamined Patent Application Publication No. 2007-100213

Patent Document 7: Japanese Patent Application Laid-Open No. 4-54734

However, the recent increase in the maximum heating temperature and prolongation of the high temperature holding time accompanied by the increase in the heat input of the welding promotes the coarsening of the structure and causes the dissolution of Mn sulfide in the HAZ during welding, and then the reprecipitation in the subsequent cooling process. Mn sulfide fine particles cause HAZ toughness to decrease by increasing the HAZ hardness due to precipitation strengthening. Therefore, since the effect of HAZ toughness improvement should be limited only by the conventional structure refinement technique, the means of obtaining more excellent HAZ toughness is calculated | required.

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

MEANS TO SOLVE THE PROBLEM In order to achieve said subject, the present inventors experimented and examined the means of obtaining the outstanding HAZ toughness by simultaneously achieving the structure refinement by suppressing (gamma) grain coarsening, and suppressing fine Mn sulfide reprecipitation. As a result, by appropriately controlling the casting process, it is possible to disperse oxides having a circle equivalent diameter of less than 2 μm, which are effective for suppressing γ grain coarsening, at high density, and to the concentrations of Mn, REM, and Ca, which form compounds in steel. It was found that fine Mn sulfide reprecipitation is suppressed by controlling the parameter (X value described later) indicating the amount of Mn sulfide present in the steel to be less than a predetermined value on the basis. The present invention has been completed based on these findings.

That is, the high tensile strength steel plate for welding of this invention is mass% (Hereinafter, "mass%" is only described as "%"),

C: 0.02 to 0.12%,

Si: 0.40% or less (including 0%),

Mn: 1.0 to 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%

And the remainder is composed of Fe and unavoidable impurities. In the structure, Mn which forms compounds in steel is 500 / mm <2> or more of oxides with a circle equivalent diameter smaller than 2 micrometers, and 5 or less mm <2> / mm <2> and oxides with a circle equivalent diameter larger than 5 micrometers. From the concentrations of, REM and Ca, the X value defined by the formula (1) is less than 10.0.

Further, in the basic component, 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 to 0.10%), and B: 0.0010 to 0.0050% of one or more elements can be added to the chemical composition of the following (1) to (3).

(1) 1 or more types from basic component + A group

(2) 1 or more types from basic component or component + B group of said (1)

(3) the basic component, the component (1) or the component (B) + B

According to the welding high-strength thick steel sheet of the present invention, a circle equivalent diameter under a predetermined steel composition is used to reduce γ grain coarsening by using a small diameter oxide having a diameter of less than 2 μm, thereby miniaturizing the structure and at the same time as a starting point of brittle fracture. In order to suppress the formation of large-diameter oxides larger than 5 µm and to suppress the reprecipitation of fine Mn sulfides, the HAZ toughness superior to the conventional heat input welding can be obtained.

The high tensile strength steel sheet for welding according to the embodiment of the present invention will first be described in detail below from the structure conditions.

In general, it is necessary to disperse fine inclusion particles at a high density for suppressing γ grain coarsening. As the particle size increases, the number density of inclusion particles decreases, and thus γ grain coarsening cannot be suppressed. Therefore, the present inventors have determined experimentally the inclusion particle size and number density from which sufficient γ grain coarsening inhibitory effect is obtained, and it is found that γ grain coarsening is suppressed by dispersing an oxide having a circle equivalent diameter of less than 2 μm or more by 500 pieces / mm 2 or more. found. When there are less than 500 pieces / mm <2> of oxides whose circle equivalent diameter is smaller than 2 micrometers, a required gamma grain coarsening will not be obtained. Moreover, it is preferable that the number of oxides whose circle equivalent diameter is smaller than 2 micrometers is 80Ox / mm <2> or more.

In addition, as a means of inhibiting fine Mn sulfide reprecipitation in HAZ, the inventors focus on REM sulfide and / or Ca sulfide, which are more stable at higher temperatures than Mn sulfide, to form compounds in steel, Mn, REM, By controlling the X value defined as in the following formula (1) from the concentration of Ca to less than 10.0, it was found that the main substance of the sulfide became Ca sulfide and / or REM sulfide, and reprecipitation of fine Mn sulfide in HAZ was suppressed. .

X = 100 × [insol Mn] / ([insol REM] + [insol Ca] +0.05) ... (1)

However, [insol Mn], [insol REM], and [insol Ca] represent the concentration (%) of Mn, REM, and Ca which form a compound in steel, respectively.

The technical meaning which the said X value represents is as follows. X value is a parameter expressing the amount of Mn sulfide present in steel. About Mn sulfide, the number or size of Mn sulfide particle | grains is generally evaluated by the structure observation using an optical microscope, a scanning electron microscope, etc. However, in the component system of the present invention, Mn sulfides are rarely present alone, and since most of them exist as composite particles with REM sulfides, Ca sulfides, and the like, it is difficult to determine the amount of Mn sulfides by observation of the structure. Therefore, in this invention, the method of indirectly evaluating Mn sulfide amount by X value was employ | adopted. That is, when the X value is less than 10.0, the amount of REM and Ca present as a compound is sufficiently secured with respect to the amount of Mn present as a compound, so that the main body of the sulfide becomes a more stable REM sulfide and Ca sulfide at high temperature, and the fine in HAZ. Reprecipitation of Mn sulfide is suppressed. When the X value is 10.0 or more, the main substance of sulfide in the steel is Mn sulfide, and reprecipitation of fine Mn sulfide is not sufficiently suppressed. Moreover, it is preferable that X value is less than 8.0.

In order to obtain good HAZ toughness, in addition to the above-described fine oxide dispersion and fine Mn sulfide reprecipitation suppression, it is necessary to control oxides having a circle equivalent diameter larger than 5 µm in steel to 5 pieces / mm 2 or less. When more than 5 oxides / mm <2> of oxides whose circular equivalent diameter is larger than 5 micrometers exist, these oxides act as a starting point of brittle fracture, and toughness deteriorates. Moreover, it is preferable that the oxide whose circle equivalent diameter is larger than 5 micrometers is 3 pieces / mm <2> or less.

In order to obtain the distribution of the oxide and the X value, it is necessary to control the amount of dissolved oxygen, the amount of alloying of REM and / or Ca, and Zr, the time from the completion of the addition of the alloying element to the start of injection in the casting step as follows. That is, at the time of casting, Mn, Si, Al, and the like are added prior to the addition of Ti, and the amount of dissolved oxygen before the addition of Ti is controlled to 0.0020 to 0.0100%, based on the amount of dissolved oxygen and the amount of sulfur before the addition of Ti (2) After determining the addition amount of REM and / or Ca and Zl so that the Z value obtained from the above is 0.58 or more, REM and / or Ca and Zr are added after the addition of Ti, and from the completion of addition of these alloy elements to the start of injection After maintaining the time t (min) in the range defined by the following formula (3), the cooling time at 1450 to 1500 ° C. during which solidification proceeds during casting 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

In addition, [O] and [S] are the dissolved O amount and the dissolved S amount before Ti addition, respectively, expressed as%, and [REM], [Ca], and [Zr] are the addition amounts of REM, Ca, and Zr represented by%, respectively. to be.

At the time of casting, if the amount of dissolved oxygen before Ti addition is less than 0.0020%, the amount of oxide having a circle equivalent diameter smaller than 2 µm cannot be sufficiently secured. In addition, when the amount of dissolved oxygen is greater than 0.0100%, or when REM and / or Ca and Zr are added before Ti, the amount of oxide having a circle equivalent diameter of more than 5 μm is increased, or the circle equivalent diameter is more than 2 μm. The amount of small oxides cannot be obtained sufficiently.

The Z value is a value considering the amount of REM, Ca, and S that contributes to the formation of REM sulfide and / or Ca sulfide. If the Z value is less than 0.58, the ratio of S to REM and / or Ca is too high. Increasing the amount of Mn sulfide produced, or the amount of solid solution S in, increases the amount of reprecipitation of fine Mn sulfide in HAZ. Each coefficient in the formula was experimentally determined with respect to the above-described formula of Z value and t (min).

In addition, when the time t (min) from the completion of the alloy element addition to the start of the injection deviates from the range of the above Equation (3), a sufficient HAZ toughness improvement effect cannot be obtained. That is, when the value of t becomes 3 + 200 × [O] × [S] / ([REM] + [Ca]) or less, the formation of REM sulfide and / or Ca sulfide in molten steel does not proceed sufficiently, so that fine Reprecipitation of Mn sulfide is not fully suppressed. Moreover, when the value of t becomes 60 or more, oxides larger than 5 micrometers of circular equivalent diameters increase by coalescence and growth of oxides, and it becomes a cause of a fall of HAZ toughness.

In addition, the time for forming REM sulfides and / or Ca sulfides as secondary inclusions if the cooling time when passing the temperature of 1450 to 1500 DEG C is shorter than 60 s after injecting the component-adjusted molten metal into the mold and then solidifying. Since this is not sufficiently secured, the amount of Mn sulfide produced at a lower temperature or the amount of solid solution S is increased to increase the amount of reprecipitation of fine Mn sulfide in the HAZ. In addition, if the cooling time exceeds 300s, the formation of an oxide having a circle equivalent diameter of more than 5 mu m in the alloy element concentration region due to solidification segregation is promoted, resulting in a decrease in the HAZ toughness.

Next, the basic component of the chemical composition of the high tensile strength steel plate for welding which concerns on embodiment is demonstrated.

C: 0.02 to 0.12%

C is an element essential for ensuring strength, and if the content is less than 0.02%, the required strength cannot be obtained, so the lower limit is set to 0.02%. In addition, when content exceeds 0.12%, since the HAZ toughness fall by hard MA structure increase, the upper limit was made into 0.12%. Moreover, a preferable minimum is 0.04% and a preferable upper limit is 0.10%.

Si: 0.40% or less (including 0%)

Si is an element which secures strength by solid solution strengthening, and when the content exceeds 0.40%, the hard MA structure (mixture structure of martensite and residual austenite) increases, causing a decrease in HAZ toughness, so the upper limit is 0.40. It was made into%. Moreover, Preferably it is 0.35% or less (including 0%).

Mn: 1.0 to 2.0%

Mn is an element effective for securing strength, and when the content is less than 1.0%, the required strength cannot be obtained, so the lower limit is 1.0%. Moreover, when content exceeds 2.0%, the excessive increase of HAZ intensity | strength will be caused, and a cause of the HAZ toughness falls, and the upper limit was made into 2.0%. Moreover, a preferable minimum is 1.4% and a preferable upper limit is 1.8%.

P: 0.03% or less (including 0%)

P is an impurity element which causes grain boundary destruction by grain boundary segregation, and when content exceeds 0.03%, HAZ toughness is caused, the upper limit was made 0.03%. Moreover, it is preferably 0.02% or less (including 0%).

S: 0.015% or less (including 0%)

S is an element which causes HAZ toughness reduction by reprecipitation of fine Mn sulfide in HAZ by being present as Mn sulfide. When the content exceeds 0.015%, reprecipitation of Mn sulfide is not suppressed, so the upper limit is 0.015%. I did it. Moreover, Preferably it is 0.012% or less (including 0%).

Al: 0.050% or less (including 0%)

Al is an element used for deoxidation during casting. When the content exceeds 0.050%, Al forms a coarse oxide and causes a decrease in the HAZ toughness, so the upper limit is made 0.050%. Moreover, Preferably it is 0.040% or less.

Ti: 0.005 to 0.100%

Ti is an element contributing to the formation of the fine oxide by being added prior to REM and Zr. When the content is less than 0.005%, a sufficient effect cannot be obtained, so the lower limit is set to 0.005%. Moreover, when content exceeds 0.100%, since coarsening of an oxide will cause HAZ toughness fall, the upper limit was made into 0.100%. The lower limit is preferably 0.01%, the upper limit is preferably 0.080%, more preferably 0.060%, and still more preferably 0.050%.

REM: 0.0002 to 0.0500% and / or Ca: 0.0003 to 0.0100%

REM (rare earth elements) and Ca form REM sulfide and Ca sulfide, respectively, to reduce the amount of Mn sulfide and to suppress the decrease in HAZ toughness due to reprecipitation of fine Mn sulfide. In order to fully acquire these effects, it is necessary to contain 0.0002% or more in REM and 0.0003% or more in Ca. In addition, it is preferably 0.0005% or more in REM and 0.010% or more in Ca. However, when the content of these elements becomes excessive, the HAZ toughness decreases due to the formation of coarse oxide, so it is necessary to set it to 0.0500% or less in REM and 0.0100% or less in Ca. Moreover, Preferably it is 0.0400% or less in REM and 0.0080% or less in Ca.

Zr: 0.0001 to 0.0500%

Zr is an element that contributes to the formation of fine oxides having a circle equivalent diameter smaller than 2 μm by addition after Ti addition during casting. When the content is less than 0.0001%, the effect cannot be sufficiently obtained. I did it. Moreover, when content is more than 0.0500%, coarse oxide or fine carbide which causes precipitation strengthening is formed and a toughness is reduced, and the upper limit was made 0.0500%. Moreover, a preferable minimum is 0.0005% and a preferable upper limit is 0.0400%.

N: 0.0020 to 0.0300%

N is an element which forms Ti nitride and raises toughness. When the content is less than 0.0020%, sufficient effect cannot be obtained, so the lower limit is set to 0.0020%. Moreover, when content exceeds 0.0300%, since toughness fall by distortion aging as solid solution N, the upper limit was made 0.0300%. Moreover, a preferable minimum is 0.0030%, a preferable upper limit is 0.0250%, a more preferable upper limit is 0.0200%, and a more preferable upper limit is 0.0150%.

O: 0.0005 to 0.0100%

O is an essential element for the production of fine oxide, and if the content is lower than 0.0005%, a sufficient amount of oxide cannot be obtained, so the lower limit is set to 0.0005%. Moreover, when content exceeds 0.0100%, coarsening of an oxide will cause HAZ toughness fall, and the upper limit was made into 0.0100%. Moreover, a preferable minimum is 0.0010% and a preferable upper limit is 0.0080%.

Group A (Ni: 0.05-1.50%, Cu: 0.05-1.50%, Cr: 0.05-1.50%, Mo: 0.05-1.50%) to the basic component to further improve the mechanical properties of the steel sheet relative to the basic component, The composition of the following (1) to (3) (the remainder is Fe and inevitable impurities) by adding one or more kinds of group B (Nb: 0.002 to 0.10%, V: 0.002 to 0.10%) and B: 0.0010 to 0.0050%. You can do

(1) 1 or more types from basic component + A group

(2) 1 or more types from basic component or component + B group of said (1)

(3) Basic component, component + B in any one of said (1) or (2)

Ni, Cu, Cr, and Mo are all effective elements for high strength of steel materials, and when the content was less than 0.05%, respectively, the effect could not be sufficiently obtained, so the lower limit was made 0.05%. Moreover, when content exceeds 1.50%, respectively, excessive increase of intensity | strength leads to a fall of toughness, and the upper limit was made into 1.50%. In addition, a preferable minimum is 0.10%, respectively, and a preferable upper limit is 1.20%, respectively.

Nb and V are elements which suppress austenite grain coarsening by being precipitated as carbonitrides. When the content is less than 0.002%, respectively, the effect is not sufficiently obtained, so the lower limit is set to 0.002%. In addition, when content exceeds 0.10%, toughness will be reduced as coarse carbonitride, respectively, and the upper limit was 0.10%. Moreover, a preferable minimum is 0.005% and a preferable upper limit is 0.08%, respectively.

B is an element which improves toughness by suppressing grain boundary ferrite production. When the content is less than 0.0010%, the effect cannot be sufficiently obtained, so the lower limit is set to 0.0010%. Moreover, when content is more than 0.0050%, it precipitates in austenite grain boundary as BN, and since toughness falls, the upper limit was made into 0.0050%. Moreover, a preferable minimum is 0.0015% and a preferable upper limit is 0.0040%.

Next, the manufacturing method of the said high tension thick steel plate for welding is demonstrated. The characteristic in the manufacturing method of the said thick steel plate exists in the casting process, Since this was already demonstrated, the process with respect to the steel ingot after casting is demonstrated easily. The ingot produced by satisfying the casting process, component range and the like was subjected to a rolling start temperature of about 1200 to 900 ° C. and a rolling end temperature of about 950 to 750 ° C. according to a usual hot rolling order, and after completion of rolling, room temperature to A thick steel plate is obtained by cooling to the cooling stop temperature of about 500 degreeC. You may further perform a tempering process after completion | finish of cooling. In addition, the thickness of the thick steel sheet to be manufactured is not particularly limited, but about 50 to 120 mm is required as the thick steel sheet for welding, and the present invention can obtain excellent HAZ toughness even with such a sheet thickness.

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

<Example>

After adjusting the addition amount of silicon or the like by using a vacuum melting furnace (150 kg), the amount of dissolved oxygen before Ti addition was changed to determine the addition amount of REM and / or Ca and Zr while considering the Z value. Except for No. 26), after adding Ti, REM and / or Ca, Zr were added, and the time t (min) from the completion of the alloying element addition to the start of the injection and the injection of the component adjusted melt were injected into the mold. During the solidification, the steel is melted by changing the cooling time t1 (s) when passing through a temperature range of 1450 to 1500 ° C, and the obtained ingot has a rolling start temperature of about 950 ° C and a final rolling temperature of about 880 ° C. To obtain a thick steel sheet having a thickness of 80 mm. In Table 1 and Table 2, based on the dissolved oxygen amount [O] (%) and Z value before Ti addition, and (3) Formula, the permissible lower limit of t (min) represented by following formula (4) Y, t ( Table 3 shows each value of min) and t1 (s).

Y = 3 + 200 × [O] × [S] / ([REM] + [Ca]) ... (4)

In addition, [O] and [S] are the dissolved O amount (%) and dissolved S amount (%) before Ti addition, respectively, and [REM] and [Ca] are the addition amount (%) of REM and Ca, respectively.

At t (plate thickness) / 4 position of each obtained thick steel plate, the density | concentration of Mn, REM, and Ca which form a compound was measured, and X value was computed. The concentration of Mn forming the compound was measured by the oxo methanol method. In addition, the concentration of REM and Ca which forms a compound was measured by the electrolytic extraction method using the solution containing 2 cc of triethanolamine and 1 g of tetramethylammonium chlorides in 100 cc of methanol as electrolyte solution. In addition, the filter of pore size 0.1 micrometer was used at the time of filtration of a residue. The obtained X value is shown in Table 3.

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

First, the observation magnification of FE-SEM was set to 5000 times, and a field of view having an area of 0.0024 mm 2 was randomly selected from 20 fields to capture images of each field of view. At the same time, the central portion of individual inclusion particles having a maximum diameter of 2 µm or less included in each field of vision was measured by EDS attached to FE-SEM, and the inclusion particles containing oxygen in the constituent elements were determined to be oxides. In addition, since the reliability of EDS measurement was low about the inclusion particle of 0.2 micrometer or less in the largest diameter, it excluded from the measurement object. Thereafter, the obtained image is subjected to image analysis using image processing software (software name: Image-Pro Plus, manufactured by Media Cybernetic), and the number density N1 of the oxide equivalent diameter of these oxides smaller than 2 µm (pcs / mm2). ) The value of N1 is shown in Table 3. Similarly, Table 3 shows the value of the number density NL (pieces / mm 2) of the oxide whose circle equivalent diameter is larger than 5 μm, calculated from 20 fields having an area of 0.06 mm 2 with the observation magnification of FE-SEM set to 1000 times. .

Moreover, the Charpy test piece (V notch) was extract | collected in parallel with the rolling direction from t (plate thickness) / 4 position of each obtained thick steel plate, and the HAZ heat cycle at the time of high heat input welding was simulated. The toughness when the reproduction HAZ heat cycle was performed was evaluated. The reproducing HAZ thermal cycle is a test specimen heated to 1400 ° C. and held for 60 s, followed by cooling the temperature range of 800 to 500 ° C. over 500 s. Charpy impact test is the HAZ toughness to a minimum value vE _-40 (min) of the measure of the impact absorbed energy vE _-40 (J) at -40 ℃ with respect to three test pieces in accordance with JIS Z 2242 is more than 100J Rated excellent. Table 3 shows the values of vE- ₄₀ (min).

As is clear from Tables 1 to 3, Inventive Examples Nos. 1 to 25 properly controlled the composition and the casting process of the thick steel sheet, so that the oxide was dispersed in an appropriate form, and the X value was less than 10.0. For this reason, tissue refinement | miniaturization and fine Mn sulfide reprecipitation suppression can be achieved, and the high value in HAZ toughness [vE- ₄₀ (min)] was obtained. On the other hand, in Comparative Examples Nos. 26 to 47, the order of addition of Ti, REM, and Zr, the dissolved oxygen amount [O] (%) and Z value before the addition of Ti, the time t (min) from the completion of the alloy element addition to the start of the injection and the injection After that, because the cooling time t1 (s) when passing through the temperature range of 1450-1500 ° C., or the composition of the steel deviated from an appropriate range, the prescribed oxide form could not be obtained, or the X value exceeded 10.0. Therefore, the HAZ toughness is considerably lowered compared to the invention example due to the increase of coarse inclusions, increase of impurities, excessive strengthening, and grain boundary segregation of solid solution elements.

Claims (5)

In mass%, C: 0.02 to 0.12%, Si: 0.40% or less (including 0%), Mn: 1.0 to 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%, At least one of REM: 0.0002 to 0.0500% and 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 remainder being a steel composed of Fe and unavoidable impurities, wherein oxides having a circle equivalent diameter of less than 2 μm in the steel and having an equivalent circle diameter of not less than 2 μm / mm 2 and larger than 5 μm are 5 High tensile strength steel sheet for welding excellent in the toughness of the heat affected zone during high heat input welding, which is not more than 2 mm2 or less and the X value defined by the formula (1) is less than 10.0 from the concentrations of Mn, REM, and Ca forming the compound in steel. . X = 100 × [insol Mn] / ([insol REM] + [insol Ca] +0.05) ... (1) However, [insol Mn], [insol REM], and [insol Ca] represent the concentration (mass%) of Mn, REM, and Ca which form a compound in steel, respectively. The method according to claim 1, wherein in mass%, Ni: 0.05-1.50%, Cu: 0.05-1.50%, Cr: 0.05-1.50%, Mo: 0.05-1.50%, High tensile steel plate for welding further comprising one or two or more of them. The method according to claim 1, wherein in mass%, Nb: 0.002 to 0.10%, V: 0.002 to 0.10% High tensile steel plate for welding further comprising one or two or more of them. The method according to claim 2, wherein in mass% Nb: 0.002 to 0.10%, V: 0.002 to 0.10% High tensile steel plate for welding further comprising one or two or more of them. The mass% according to any one of claims 1 to 4, wherein B: High tensile strength steel sheet for welding, further comprising 0.0010 to 0.0050%.