JP2011080156A - Thick high strength steel plate having excellent low temperature toughness in welding heat affected zone caused by high heat input welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick high strength steel plate which can achieve excellent welding HAZ toughness, even when a welding is carried out with a welding heat input of 20 to 100 kJ/mm on a steel plate having a thickness of 50 to 80 mm and having a base material tensile strength of about 490 to 570 MPa. <P>SOLUTION: The steel plate has a chemical composition, in mass%, of C: 0.03 to 0.14%, Si: 0.30% or less, Mn: 0.8 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ni: 1.5 to 4.0%, Nb: 0.003 to 0.040%, Al: 0.001 to 0.040%, N: 0.0010 to 0.0100%, Ti: 0.005 to 0.030%, in which the following Ceq satisfies 0.36 to 0.42 and the contents of Ni and Mn satisfy following formula [1], and the balance: iron and inevitable impurities. [1] Ni/Mn≥10×Ceq-3, wherein Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a thick high-strength steel sheet excellent in low temperature toughness of a weld heat affected zone (hereinafter referred to as HAZ) used for ships, offshore structures, middle-high-rise buildings, bridges, and the like. Further, the present invention relates to a steel plate having an excellent weld joint even when welding with a welding heat input of 20 to 100 kJ / mm is performed on a steel plate having a thickness of 50 mm or more and a base metal tensile strength of 490 to 570 MPa.

  In recent years, demands for material properties of steel materials for welding used in large structures such as ships, offshore structures, high-rise buildings, and bridges have increased. In particular, in these structures, the use of steel plates having a thickness exceeding 50 mm and a tensile strength of the base material of 570 MPa class is increasing. Also, in order to promote the efficiency of welding, one-pass welding by high heat input welding methods such as electrogas welding method and electroslag welding method is considered for welding such thick high strength steel plates. As with the toughness of the base material itself, the demand for HAZ toughness is becoming stricter.

  Many proposals that focus on the HAZ toughness of steel materials to which the high heat input welding method is applied have been made so far. For example, Patent Document 1 discloses an invention that reduces the austenite grains of HAZ and secures toughness by securing fine Ti nitride in steel. Patent Document 2 proposes an invention in which a composite precipitate of Ti nitride and MnS is utilized as a ferrite transformation nucleus to improve the toughness of HAZ. Furthermore, Patent Document 3 proposes an invention in which a composite precipitate of Ti nitride and BN is utilized as a precipitation nucleus of grain boundary ferrite to improve HAZ toughness.

  However, since this Ti nitride is almost dissolved in the vicinity of the boundary (hereinafter also referred to as a weld bond portion) with the weld metal having a maximum ultimate temperature exceeding 1400 ° C. of HAZ, the effect of improving toughness is reduced. There is a problem that it ends up. For this reason, it is difficult for steel materials using Ti nitride as described above to achieve the strict requirements for HAZ toughness in recent years and the required characteristics of HAZ toughness in super-high heat input welding.

  As a method for improving the toughness in the vicinity of the weld bond portion, steel containing Ti oxide is used in various fields such as thick plates and section steel. For example, in the field of thick steel plates, as in the inventions described in Patent Document 4 and Patent Document 5, steel containing Ti oxide is very effective in improving the toughness of large heat input welds. Application is promising. This principle is based on the fact that Ti oxide, which is stable even at the melting point of steel, is used as a precipitation site, Ti nitride, MnS, etc. are precipitated in the middle of the temperature drop after welding, and further, fine ferrite is generated using them as a site. The generation of coarse ferrite harmful to toughness is suppressed, and deterioration of toughness can be prevented.

  However, such a Ti oxide has a problem that the number dispersed in steel cannot be increased too much. The cause is considered to be coarsening or aggregation of Ti oxides, and if the number of Ti oxides is increased, coarse Ti oxides of 5 μm or more, so-called inclusions are increased. This inclusion of 5 μm or more should be avoided because it is harmful because it becomes a starting point of destruction of the structure or causes a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it is necessary to utilize an oxide that is less likely to be coarsened or aggregated and is more finely dispersed than Ti oxide.

  Further, as a method of dispersing such Ti oxide in steel, there are many methods by adding Ti to molten steel which does not substantially contain a strong deoxidizing element such as Al. However, it is difficult to control the number of Ti oxides and the degree of dispersion in the steel simply by adding Ti to the molten steel. Further, the number and the degree of dispersion of precipitates such as TiN and MnS are controlled. It is also difficult. Therefore, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, the number of Ti oxides cannot be obtained sufficiently, or the toughness in the thickness direction of the thick plate varies. .

  With respect to such a problem, in Patent Document 6 and Patent Document 7, Ti—Al composite oxide or composite oxide of Ti, Al, and Ca that is generated by Al addition immediately after Ti addition or Al / Ca composite addition is disclosed. An invention utilizing the above is disclosed. By such an invention, it became possible to greatly improve the high heat input welding HAZ toughness.

Japanese Patent Publication No. 55-026164 Japanese Patent Laid-Open No. 03-264614 Japanese Patent Laid-Open No. 04-143246 JP 61-079745 A JP-A-62-103344 Japanese Patent Laid-Open No. 06-293937 JP-A-10-183295

  However, in the above conventional means for reducing the HAZ austenite grains or generating ferrite using precipitates as ferrite transformation nuclei, in order to ensure a base material strength of 490 MPa or more with a plate thickness of 50 mm or more. It is necessary to increase the alloy elements. In this case, the hardness of the welded HAZ is increased, and the generation of MA (Martensite-Austenite constituent) that deteriorates toughness becomes obvious. Sufficient HAZ toughness such as grade (guaranteed at −20 ° C.) cannot be secured stably. Furthermore, if the base material strength is 570 MPa or more in terms of tensile strength, the required HAZ toughness cannot be obtained.

  Therefore, the present invention is a steel plate having a thickness of 50 to 80 mm and a base material tensile strength of 490 to 570 MPa, and can achieve excellent weld HAZ toughness even when welding heat input is 20 to 100 kJ / mm. It is an object of the present invention to provide a thick high strength steel plate excellent in low temperature toughness of a weld heat affected zone by high heat input welding.

The present inventors have found that the above problems can be advantageously solved by defining the amount of Ni added and Ni / Mn, and the present invention has been completed for the first time after further studies. Is as follows.
(1) In mass%,
C: 0.03-0.14%, Si: 0.30% or less,
Mn: 0.8 to 2.0%, P: 0.02% or less,
S: 0.005% or less, Al: 0.001-0.040%,
N: 0.0010 to 0.0100%, Ni: 1.5 to 4.0%,
Ti: 0.005-0.030%, Nb: 0.003-0.040%
The following Ceq is 0.36 to 0.42, Ni and Mn satisfy the formula [1], and the balance is iron and inevitable impurities. Thick high-strength steel sheet with excellent low temperature toughness.
Ni / Mn ≧ 10 × Ceq-3 [1]
However, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 1 5
(2 ) Furthermore, in mass%,
B: 0.0005 to 0.0050%
A thick high-strength steel sheet excellent in low-temperature toughness of the weld heat-affected zone by high heat-input welding as described in (1 ) above.
( 3 ) Furthermore, in mass%,
Cr: 0.1-0.5%, Mo: 0.01-0.5%,
V: 0.005-0.10%, Cu: 0.1-1.0%
A thick, high-strength steel sheet excellent in low temperature toughness of the weld heat-affected zone by high heat input welding as described in (1) or (2) above, comprising one or more of them.
(4) Furthermore, in mass%,
B: 0.0005 to 0.0050%
In addition, in mass%,
Cr: 0.1-0.5%, Mo: 0.01-0.5%,
V: 0.005-0.10%, Cu: 0.1-1.0%
1 type or 2 types or more, and further, in mass%,
Ca: 0.0003 to 0.0050%, Mg: 0.0003 to 0.0050%,
REM: 0.001 to 0.030%
One or more of these, and
O: 0.0010 to 0.0050%
And an oxide having an equivalent circle diameter of 0.005 to 0.5 μm is contained at 100 pieces / mm ² or more, and the heat affected zone by high heat input welding according to (1) above Thick high-strength steel sheet with excellent low-temperature toughness.

  The present invention supplies thick steel plates that satisfy severe toughness requirements for the destruction of ships, offshore structures, mid-to-high-rise buildings, etc., and the effects brought to this type of industrial field are extremely great. The contribution to society is very significant from the meaning.

It is a figure which shows the welding thermal cycle equivalent to 45 kJ / mm. It is a figure which shows the relationship between Ni / Mn, Ceq, and reproduction HAZ toughness. It is a figure which shows the reproduction HAZ toughness improvement effect by fine oxide dispersion | distribution or B utilization.

The present invention is described in detail below.
So far, as described above, as a means for improving the HAZ toughness, it has been considered to suppress the growth of austenite grains at a high temperature. The most effective method for this is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. This is because even when the welding heat input is as large as 20 to 100 kJ / mm, the HAZ reheated austenite grains are refined very effectively by pinning. However, in order to increase the strength of the base metal, the alloy addition amount is increased, and in the steel material in which the carbon equivalent (Ceq) showing the chemical component hardenability simultaneously with the weldability of the steel material is 0.36 or more, the hardness of the HAZ is increased. Therefore, there is a new problem that sufficient HAZ toughness cannot be obtained even when reheated austenite grains are refined by pinning. Thus, when the hardness of a HAZ part becomes high, it is necessary to improve the toughness of the base iron itself.

  Therefore, the inventors are the optimum component system for improving the toughness of the base iron itself in order to improve the HAZ toughness when Ceq is as high as 0.36 or more and 0.42 or less, which is necessary for the thick high-strength steel in question. We have studied earnestly. It has been conventionally known that Ni is effective as an element for increasing the toughness of the matrix. However, as in this case, it is not known whether it is effective for improving the toughness of HAZ having a high Ceq of 0.36 to 0.42, and if it is effective, what component conditions are effective. Therefore, first, the influence of the Ni addition amount was examined. In the examination, it was assumed that 0.003% or more of Nb amount effective for securing the strength of the base material was added. For the evaluation of HAZ toughness, the ductile / brittle transition temperature (vTrs) in the Charpy impact test when a thermal cycle corresponding to electrogas welding (heat input 45 kJ / mm) shown in FIG. 1 was applied was adopted.

As a result of examining the influence of the amount of Ni added, it was first found that the necessary toughness could not be obtained when Ni was less than 0.8%. In addition, even when Ni was added in an amount of 0.8% or more, there was a case where the HAZ toughness was not improved and a case where the HAZ toughness was decreased. Therefore, as a result of intensive studies including the relationship with other additive elements and Ceq, as shown in FIG. 2, when Ceq is 0.36 or more and 0.42 or less, the HAZ toughness is as follows. / Mn and found to be related. FIG. 2 is a plot of the reproducible HAZ toughness (vTrs) of the steel used in the study, stratified for each Ceq, with the Ni / Mn ratio as the horizontal axis. From FIG.
Ni / Mn ≧ 10 × Ceq-3 [1]
In the steel material satisfying this relationship, good toughness of −15 ° C. or lower was obtained in vTrs. The reason why the steel material that does not satisfy the formula [1] cannot obtain sufficient HAZ toughness is that the addition amount of Ni is not sufficient and the effect of toughening the matrix is small, or even if it contains a large amount of Ni, Mn It is considered that MA is formed in the HAZ by excessive addition of Ni and the toughening effect of Ni disappears. In addition, as a result of carrying out the same examination with the heat cycle equivalent to heat input of 100 kJ / mm for the steel material used in the above examination, even in the case of heat input of 100 kJ / mm, the steel material having the relationship of the formula [1] is good It has been confirmed that a reproducible HAZ toughness can be obtained.

  As a result of the above study, the HAZ toughness was found to be improved by adding 0.8% or more of Ni that satisfies the formula [1], but the inventors further studied the improvement of the HAZ toughness. The following three methods were examined as methods for improving HAZ toughness. First, in high heat input welding, since the high temperature residence time is prolonged, the austenite grains become coarse and this reduces the HAZ toughness. Therefore, this is a method of suppressing the austenite coarsening during high temperature residence. Secondly, in high heat input welding, since the cooling time after welding is long, the ferrite formed from the austenite grain boundaries becomes coarse, and this coarse grain boundary ferrite causes the HAZ toughness to decrease. This is a method of suppressing the conversion. Third, the HAZ structure itself is made fine.

As a method for suppressing the coarsening of the first austenite grains, for example, as described in Patent Document 7, a method of dispersing a fine oxide is effective. In Patent Document 7, the amount of dissolved oxygen in a molten steel is adjusted by an equilibrium reaction with Si in a deoxidation process for fine oxide dispersion, and then deoxidized in the order of Ti, Al, and Ca. And by this method, an oxide having a particle diameter of 0.01 to 1.0 μm is dispersed at 5 × 10 ³ to 1 × 10 ⁵ particles / mm ² .

Therefore, the inventors dispersed HAS in a system in which Ceq is as high as 0.36 or more and 0.42 or less, and Nb is contained in an amount of 0.003% and Ni is added in an amount of 0.8% or more. A method for further improving the toughness was intensively studied. First, a method of dispersing fine oxides. In such a system, the amount of dissolved oxygen in the molten steel is adjusted to 0.0010 to 0.0050% in the deoxidation step, and then first deoxidized with Ti. Subsequently, after deoxidizing with Al, one or more of Ca, Mg, and REM are further added to disperse 100 oxides / mm ² or more of fine oxides having an equivalent circle diameter of 0.005 to 0.5 μm. I found that it was possible.

Moreover, this fine oxide dispersion could suppress the austenite grain coarsening at the time of high temperature residence in welding and further improve the HAZ toughness. As an example, the result compared with the HAZ toughness which added only Ni appropriately is shown in FIG. Note that the generated oxide is finer and more numerous as the amount of Ni increases. When the amount of Ni is 1.5% or more, the oxide is 1000 / mm ² or more. This was discovered this time. Furthermore, with regard to the amount of Si in the molten steel, since it is difficult to form an oxide when the amount of Si is large, it is preferable that the amount of Si is 0.30% or less, more preferably 0.20% or less. It became clear from.

  On the other hand, when the amount of dissolved oxygen before Ti deoxidation exceeds 0.0050% or when the order of deoxidation elements is different, the oxide becomes coarse and fine oxide cannot be obtained sufficiently. Almost no inhibitory effect is obtained.

In addition, the number of oxides having an equivalent circle diameter of 0.005 to 0.5 μm was obtained by producing an extraction replica from a steel plate as a base material, and 10000 times with an electron microscope at least 100 fields of view (observation area 10000 μm ² The above was observed, and the particles of less than 0.1 μm were observed with the magnification increased appropriately. Elemental analysis was performed on the observed particles having a diameter of 0.005 to 0.5 μm, and the number of oxide particles was counted.

  Next, the inventors diligently studied the coarsening suppression of grain boundary ferrite and the refinement of the HAZ structure described as the second method and the third method as the HAZ toughness improving method. . As a result, when Ceq is as high as 0.36 or more and 0.42 or less, and in a system to which Ni is added at 0.8% or more, particularly when performing high heat input welding equivalent to 20 to 100 kJ / mm like this time In addition, it was found that addition of B was effective. This is because, in terms of suppressing the coarsening of the grain boundary ferrite, the formation of grain boundary ferrite is suppressed by the segregation of the solid solution B at the reheated austenite grain boundary.

  In addition, in terms of miniaturization of the HAZ structure, in the case where the cooling rate is low in the large heat input welding as in this time, B nitride precipitates on the austenite grain boundaries and inclusions in the austenite grains by adding B, This is because the HAZ structure is refined by forming a large number of fine ferrite having a diameter of several μm at the austenite grain boundaries and in the grains. FIG. 3 shows the result of comparing the improvement of HAZ toughness by addition of B with the HAZ toughness of only adding Ni appropriately. It can be seen that the addition of B further improves the HAZ toughness. Further, FIG. 3 shows the HAZ toughness when B is added in addition to the above-described method of dispersing the fine oxide, but the HAZ toughness is further improved by the fine oxide dispersion and addition of B. This is thought to be due to the fact that the number of oxides serving as BN precipitation sites increased, the ferrite that nucleates the BN increased, and the HAZ structure further refined.

  In addition to the above conditions, HAZ toughness when Cu, Cr, Mo, and V were added was also examined from the viewpoint of securing strength and improving corrosion resistance. As a result, if the addition is within the range of 0.1 to 0.4%, 0.1 to 0.5%, 0.03 to 0.2%, and 0.005 to 0.050%, HAZ It has been found that toughness is not significantly reduced.

  In addition, the manufacturing method of the steel plate of this invention is not restrict | limited in particular, What is necessary is just to manufacture according to a well-known method. For example, after the molten steel adjusted to the above-mentioned preferred component composition is made into a slab by a continuous casting method, it is heated to 1000 to 1250 ° C. and then hot rolled.

Next, the reason for limiting the component composition of the steel material used in the present invention will be described. Hereinafter, the mass in the composition is simply expressed as%.
C is an effective component for improving the strength of steel. The lower limit is 0.03%, and excessive addition generates a large amount of carbides and MA and significantly reduces the HAZ toughness. Therefore, the upper limit is 0.14%. did.

  Si is a component necessary for securing the strength of the base material, deoxidation, etc., but the upper limit was made 0.30% in order to prevent the toughness from being lowered by the hardening of the HAZ. Further, in the case of using an oxide, the upper limit is preferably made 0.20% or less in order to prevent a decrease in the oxygen concentration in the molten steel.

Mn needs to be added in an amount of 0.8% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is set to 2.0% within an allowable range such as toughness and crackability of the weld. . Further, regarding the upper limit of Mn, it is necessary to satisfy the formula [1] indicating the relationship between Ceq, Mn content, and Ni content. This is based on the fact that when the Ceq is high, an increase in Mn causes a large amount of MA to be generated in the HAZ structure, and the effect of improving the HAZ toughness by Ni is lost, which is newly found in this study.
Ni / Mn ≧ 10 × Ceq-3 [1]

  The content of P is preferably as low as possible. However, in order to reduce this industrially, it takes a great deal of cost, so the content range was set to 0.02 or less.

  The content of S is preferably as low as possible, but since it takes a great deal of cost to reduce this industrially, the content range is set to 0.005 or less.

Ni is an important element in the present invention and needs to be added at least 0.8%. Furthermore, regarding the lower limit of Ni , it is necessary to satisfy the equation [1] indicating the relationship between Ceq, the amount of Mn, and the amount of Ni. The upper limit was set to 4.0% from the viewpoint of manufacturing cost. The lower limit of the Ni content is set to 1.5% or more based on the examples.
Ni / Mn ≧ 10 × Ceq-3 [1]

  Nb is an effective element for improving the strength of the base material by improving the hardenability, so 0.003% or more is added. However, when a large amount of Nb is added, MA tends to be generated in the HAZ regardless of the Ni / Mn ratio, and when it is added more than 0.040%, a large number of coarse MA having a major axis of 5 μm or more is generated in the HAZ. Since the toughness may be greatly lowered, the upper limit of Nb is set to 0.040%. In order to obtain higher toughness, the Nb content should be suppressed to 0.020% or less in which the coarse MA having a major axis of 5 μm or more is hardly generated in the case of the Ni / Mn ratio satisfying the above formula [1]. preferable. In order to obtain higher toughness more stably, the amount of Nb should be suppressed to 0.010% or less where MA with a major axis of 3 μm or more hardly occurs in the Ni / Mn ratio satisfying the above formula [1]. Is preferred.

  Al is an important deoxidizing element, and the lower limit was set to 0.001%. In addition, when Al is present in a large amount, the surface quality of the slab deteriorates, so the upper limit was made 0.040%.

  Ti is added in an amount of 0.005% or more in order to generate Ti nitrides and Ti-containing oxides that serve as pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition increases the amount of dissolved Ti and causes a decrease in HAZ toughness, so 0.030% was made the upper limit.

  N adjusts the addition amount as necessary in order to generate Ti nitride and B nitride in the austenite grain boundaries and grains during cooling after welding. Addition of 0.0010% or more is necessary to form B nitride by combining with B. However, excessive addition increases the amount of dissolved N and causes a decrease in HAZ toughness. Was the upper limit.

  Ca is added in an amount of 0.0003% or more as necessary in order to generate Ca-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.0050% was made the upper limit.

  Mg is added in an amount of 0.0003% or more as necessary in order to generate Mg-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.0050% was made the upper limit.

  REM is added in an amount of 0.0001% or more as necessary in order to generate REM-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.030% was made the upper limit. Moreover, REM described here is Ce and La, and the addition amount is the total amount of both.

  B is solute B and segregates at the austenite grain boundaries during cooling after welding to suppress the formation of intergranular ferrite. Also, B is produced as necessary to produce BN within the austenite grain boundaries and grains. Add 0005% or more. However, excessive addition increases the amount of solute B, greatly increases the HAZ hardness and causes a decrease in HAZ toughness, so 0.0050% was made the upper limit.

  Cu is added in an amount of 0.1% or more as necessary to improve the strength and corrosion resistance of the steel material. The effect is saturated at 1.0%, so the upper limit was set to 1.0. However, if it exceeds 0.4%, MA tends to be formed and the HAZ toughness is lowered, so 0.4% or less is preferable. good.

  Cr is added in an amount of 0.1% or more as necessary in order to improve the corrosion resistance of the steel material, but excessive addition causes a reduction in HAZ toughness due to MA formation, so 0.5% was made the upper limit.

  Mo is an effective element for improving the strength and corrosion resistance of the base material, and is added in an amount of 0.01% or more as necessary. The effect is saturated at 0.5%, so the upper limit was made 0.5%. However, excessive addition causes a decrease in HAZ toughness due to MA formation, so 0.2% or less is preferable.

  V is an effective element for improving the strength of the base material, and is added in an amount of 0.005% as necessary. The effect is saturated at 0.5%, so the upper limit was made 0.10%. However, excessive addition causes a reduction in HAZ toughness due to MA formation, so 0.050% or less is preferable.

  A steel piece having a chemical composition shown in Table 1 was continuously cast to produce a rope piece. For D23 to D31 and D46 to D49, the dissolved oxygen in the molten steel was adjusted to 0.0010% to 0.0050% with Si before introducing Ti, and then first deoxidized with Ti and subsequently deoxidized with Al. Any of Ca, Mg, and REM was added for deoxidation. After reheating these at 1100 to 1250 ° C., steel plates having a thickness of 50 to 80 mm were manufactured by the following two types of rolling methods. One is a method in which the surface temperature is rolled in a temperature range of 750 to 900 ° C., and then the plate surface after water cooling is cooled in water until it is reheated within a temperature range of 200 to 400 ° C. (described as TMCP in Table 2). The other is a manufacturing method (indicated as DQ-T in Table 2) after hot rolling, water cooling to room temperature, and tempering in the range of 500-600 ° C.

Table 2 shows the manufacturing conditions, plate thickness, and mechanical properties of the steel plate. Moreover, regarding D23 to D31 and D46 to D49, the number of fine oxides having an equivalent circle diameter of 0.005 to 0.5 μm, which was measured at an arbitrary portion of the steel sheet, was also shown. Regarding the number of oxides, an extraction replica is prepared from an arbitrary part of the steel plate, and it is observed with an electron microscope at a magnification of 10,000 at 100 fields or more (observation area of 10,000 μm ² or more). Were observed with appropriately increased magnification.
Elemental analysis was performed on each of the particles having a diameter of 0.005 to 0.5 μm to be observed, and the number of oxide particles was counted. In each of the steel materials D23 to D31 and D46 to D49, fine oxides having an equivalent circle diameter of 0.01 to 0.5 μm are dispersed in a range of 100 / mm ² or more within the range of the present invention. From the comparison of D46, D47 and D48, D49 in which elements other than Si are almost the same, it can be seen that the smaller the amount of Si is 0.20% or less, the larger the amount of oxide.

  These steel plates were subjected to vertical one-pass welding by facing the steel plates using electrogas welding (EGW) or electroslag welding (ESW) having a welding heat input of 20 to 100 kJ / mm. And in HAZ located in a plate | board thickness center part (t / 2), notch was put into two places, 1 mm away from FL, and the Charpy impact test was done at -40 degreeC. Table 2 shows welding conditions and HAZ toughness. In this Charpy impact test, a full size test piece of JIS No. 2 mmV notch was used. Table 2 also shows the prior austenite grain size between FL and HAZ of 1 mm. The prior austenite grain size between FL and HAZ of 1 mm described here is the grain size of the prior austenite grains contained in the plane including 2 mm in the plate thickness direction centered on the plate thickness center (2 / t) and FL to HAZ of 1 mm. It is an average particle diameter measured by a cross-sectional method. Here, the measurement was performed using the massive ferrite connected in a net shape as the grain boundary of the prior austenite grains.

D1 , D2, D4, D8, D11 to D13, D15 to D31, D33, D34, D36 to D38, D40 to D45, D48, and D49 are steels of the present invention. Since the chemical composition of steel is appropriately controlled, the high heat input HAZ toughness at −40 ° C. is satisfactory while satisfying a predetermined base material performance. Further, D23 to D31 and D46 to D49 in which fine oxides are dispersed have a prior austenite grain size between FL and HAZ of 1 mm of 200 μm or less, which is finer than others, and a large heat input HAZ at −40 ° C. The toughness is even higher. In addition, D20 with the addition of B and refinement of the HAZ structure has better HAZ toughness than D19 with no addition of B and the same amount of additive elements other than B, at −40 ° C. The high heat input HAZ toughness also shows a high value.
On the other hand, C1-17 of the comparative steel does not contain sufficient Ni to satisfy the formula [1], or the chemical composition of the steel is appropriately controlled, so that the high heat input HAZ toughness is not good. It is enough.

Claims (4)

% By mass
C: 0.03-0.14%,
Si: 0.30% or less,
Mn: 0.8 to 2.0%,
P: 0.02% or less,
S: 0.005% or less,
Al: 0.001 to 0.040%,
N: 0.0010 to 0.0100%,
Ni: 1.5 ~4.0%,
Ti: 0.005 to 0.030%,
Nb: 0.003-0.040%
The following Ceq is 0.36 to 0.42, Ni and Mn satisfy the formula [1], and the balance is iron and inevitable impurities. Thick high-strength steel sheet with excellent low temperature toughness. Ni / Mn ≧ 10 × Ceq-3 [1]
However, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Furthermore, in mass%,
B: 0.0005 to 0.0050%
The thick high-strength steel sheet excellent in low temperature toughness of the weld heat affected zone by high heat input welding according to claim 1, characterized in that Furthermore, in mass%,
Cr: 0.1 to 0.5%,
Mo: 0.01 to 0.5%,
V: 0.005-0.10%,
Cu: 0.1 to 1.0%
The thick high-strength steel sheet excellent in the low temperature toughness of the welding heat-affected zone by high heat input welding according to claim 1 or 2 , characterized by containing one or more of them. Furthermore, in mass%,
B: 0.0005 to 0.0050%
In addition, in mass%,
Cr: 0.1 to 0.5%,
Mo: 0.01 to 0.5%,
V: 0.005-0.10%,
Cu: 0.1 to 1.0%
1 type or 2 types or more, and further, in mass%,
Ca: 0.0003 to 0.0050%,
Mg: 0.0003 to 0.0050%,
REM: 0.001 to 0.030%
1 type or 2 types or more, and O 2: 0.0010 to 0.0050%
The oxide having a circle-equivalent diameter of 0.005 to 0.5 μm is contained at 100 pieces / mm ² or more of the heat affected zone by high heat input welding according to claim 1, Thick high-strength steel sheet with excellent low-temperature toughness.

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