JP2013127108A - Thick steel plate excellent in toughness of weld heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate excellent in toughness of weld heat-affected zone, which can enhance a mean value and a minimum value of an HAZ toughness, even when welded at a high heat input.SOLUTION: The steel sheet satisfies a prescribed chemical composition and has an A value of 5.0-25.0 obtained from the following expression: A=10×[B]×(0.4+30×[Ti]-82×[N]). The steel sheet further includes oxides in which constituent elements except oxygen satisfy the following relations, by mass%, 2%<Ti<40%, 5%<Al<30%, 5%<Ca<40%, 5%<REM<50%, 2%<Zr<30%, 1.5≤REM/Zr. Among oxides, inclusions having a circle equivalent diameter of less than 2 μm exist 300 pieces/mmor more, inclusions having a circle equivalent diameter 2 μm or more exist 100 pieces/mmor less. Among Ti nitrides, Ti nitrides having a circle equivalent diameter of 1 μm or more exist 5 pieces/mmor below.

Description

  The present invention relates to a thick steel plate applied to a welded structure such as a bridge, a high-rise building, a ship, or a line pipe. More specifically, the present invention relates to the toughness of a heat affected zone (hereinafter also referred to as HAZ) after large heat input. It relates to an excellent thick steel plate.

  In recent years, with the increase in the size of welded structures such as bridges, high-rise buildings, ships, and line pipes, thick steel plates with a thickness of 50 mm or more are often applied to such welded structures. The welding of thick steel plates with a thickness of 50 mm or more is inevitable. In view of the above circumstances, high heat input welding for the purpose of improving welding construction efficiency is required.

  However, since HAZ during high heat input welding is kept in a high temperature austenite (γ) region by heating for a long time and then gradually cooled, γ grains grow during heating, and coarse ferrite (α) during the cooling process. The coarsening of the structure as typified by the formation of grains is likely to be caused, which causes a reduction in the toughness of the HAZ during high heat input welding. Therefore, it is necessary to develop a technique for stably maintaining the HAZ toughness (hereinafter also referred to as HAZ toughness) at the time of high heat input welding at a high level.

  Means for ensuring HAZ toughness include pinning gamma grain growth by inclusion particles such as oxides, nitrides, and sulfides, and techniques for refining the structure by intragranular α formation starting from inclusion particles. Has been proposed. As a proposal example of such a technique, there are techniques described in Patent Document 1 and Patent Document 2, and by dispersing and depositing fine Ti nitride as γ-growth growth pinning particles in a steel material, HAZ at the time of high heat input welding It suppresses the coarsening of the austenite grain which arises by this, and suppresses deterioration of HAZ toughness. However, Ti nitride tends to disappear when welding heat input is increased, and there is a problem that stable HAZ toughness cannot be obtained. Thus, Ti nitride cannot cope with the recent increase in welding heat input.

  On the other hand, Patent Documents 3 to 6 propose a technique in which oxide inclusions that are stable at high temperatures are used as γ-grown pinning particles. However, the number of oxide inclusions is smaller than that of Ti-containing nitrides, and a sufficient pinning effect cannot be obtained. Improvement is necessary.

  That is, Patent Document 3 describes that good HAZ characteristics can be obtained by the presence of an oxide containing REM or Zr, but the assumed heat input is only at a low level and is not necessarily large. No good HAZ properties can be obtained by thermal welding. Patent Document 4 describes a technique using an oxide containing REM and Zr as in Patent Document 3, and evaluates Charpy absorbed energy as HAZ toughness, but it is a viewpoint of material reliability. Therefore, it is considered necessary to guarantee not only the average value but also its minimum value to a high level.

  Furthermore, Patent Document 5 describes a technique for obtaining high HAZ toughness by using both oxide inclusions and Ti-containing inclusions as γ-growth growth pinning particles. However, considering the recent trend of increasing heat input, there is a limit to the use of Ti-containing inclusions, and it is necessary to immediately establish means for improving HAZ toughness with large heat input by oxide inclusions. Can do. In addition, the inventors have proposed a technique that utilizes the gamma grain growth pinning effect of fine oxide inclusions in Patent Document 6, but this technique is a technique that also uses reprecipitation suppression of fine Mn sulfide. Therefore, the complicated control of determining the alloy addition amount based on the dissolved oxygen amount and the dissolved sulfur amount is required.

  In addition, as a technique related to the refinement of the structure by intragranular α generation starting from inclusion particles, a technique using a composite oxide containing Ti and REM described in Patent Document 7 and MnS is proposed, Inventors have proposed the technique which accelerates | stimulates intra-granular alpha production | generation by controlling the inclusion shape in patent document 8. FIG. These techniques are constructed on the premise that inclusions with low interfacial energy are effective for intragranular α production (intragranular α / inclusions). However, in the production of intra-granular α, the contribution of (intra-granular α / γ) interfacial energy is also large, and by simply reducing (intra-granular α / inclusion) interfacial energy, sufficient intra-granular α can be obtained Therefore, sufficient heat input HAZ toughness has not been sufficiently ensured.

  Furthermore, the inventors have constructed a high HAZ toughness technique utilizing intragranular α generation starting from an oxysulfide and has proposed it as Patent Document 9. However, since it is necessary to disperse a certain number of relatively large oxysulfide particles having a size of 2 μm or more as a compensation, even this technique has not yet sufficiently secured the high heat input HAZ toughness. That is, in the technique described in Patent Document 7, the assumed heat input itself is small, and even in the techniques described in Patent Document 8 and Patent Document 9, although the average value of Charpy absorbed energy is high, the minimum value is not improved. There is room for it now.

  In addition, the inventors have proposed, as Patent Document 10 and Patent Document 11, a technique that can obtain high HAZ toughness by dispersing an oxide whose structure is controlled. Although these techniques enabled the realization of a thick steel plate with excellent weld heat affected zone toughness, problems still need to be improved in manufacturing.

  In the technique described in Patent Document 10, the Ca addition amount is controlled based on the dissolved oxygen amount before the Ca addition in order to realize a predetermined oxide form, but at the same time, the time from Ti addition to Ca addition is 3 There is a concern that the burden on the operator will increase because it is necessary to keep it within -20 minutes. On the other hand, in the technique described in Patent Document 11, since it is necessary to hold for 40 minutes to 90 minutes from the addition of Ca to the start of casting, an improvement point remains in productivity.

JP 2001-98340 A JP 2004-2181010 A Japanese Patent Laid-Open No. 2001-20031 JP 2007-247005 A JP 2008-223062 A JP 2009-179844 A Japanese Patent Laid-Open No. 7-252586 JP 2008-223081 A JP 2009-138255 A JP 2010-168644 A JP 2011-219797 A

  The present invention was made in view of the above-described conventional situation, and even when high heat input welding is performed, the average value of HAZ toughness can be improved as well as the minimum value, An object of the present invention is to provide a thick steel plate that is excellent in the toughness of the weld heat-affected zone and is also excellent in productivity.

Invention of Claim 1 is the mass%, C: 0.03-0.12%, Si: 0.25% or less (including 0%), Mn: 1.0-2.0%, P: 0.03% or less (not including 0%), S: 0.015% or less (not including 0%), Al: 0.004 to 0.05%, Ti: 0.010 to 0.050%, REM: 0.0003-0.02%, Zr: 0.0003-0.02%, Ca: 0.0005-0.010%, N: 0.002-0.010%, B: 0.0005- A thick steel plate containing 0.0050%, the balance being iron and inevitable impurities, and A = 10 ⁴ × [B] × (0.4 + 30 × [Ti] −82 × [N]) The required A value satisfies 5.0 ≦ A ≦ 25.0, and the constituent elements excluding oxygen are 2% <Ti <40%, 5% <Al in mass%. <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30%, an oxide satisfying 1.5 ≦ REM / Zr, and among the oxides , equivalent circle diameter oxides of less than 2μm is 300 / mm ² or more, circle oxides of equivalent diameter above 2μm have 100 / mm ² or less, with present, among Ti nitrides contained equivalent circle It is a thick steel plate excellent in toughness of the weld heat affected zone, characterized in that Ti nitride having a diameter of 1 μm or more is 5 pieces / mm ² or less. However, in the above formula, [] indicates the content (% by mass) of each element.

  In addition, including the above description, the equivalent circle diameter described in the present invention is the diameter of a circle assumed to have the same area, focusing on the size of the oxide and Ti nitride. It can obtain | require by observing with a transmission electron microscope (TEM) or a scanning electron microscope (SEM).

  The invention according to claim 2 further includes, in mass%, Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0. It contains at least one selected from the group consisting of 0.05 to 1.50%, and satisfies [Ni] + [Cu] + [Cr] + [Mo] <2.5%. Item 2. A thick steel plate excellent in toughness of the weld heat affected zone according to item 1. However, in the above formula, [] indicates the content (% by mass) of each element.

  The invention according to claim 3 further contains Nb: 0.002 to 0.10% and / or V: 0.002 to 0.10% by mass%. It is a thick steel plate excellent in the toughness of the described weld heat affected zone.

  According to the present invention, the average value and the minimum value of the HAZ toughness can be improved not only for small to medium heat input welding but also for large heat input welding. In addition, it is possible to obtain a thick steel plate having excellent productivity.

  The inventors searched for a means for improving the high heat input HAZ toughness of a thick steel plate under production conditions with relatively high productivity. As a result, it is possible to ensure the formation of intra-granular α at the oxide starting point, and suppress the formation of coarse Ti nitride and coarse grain boundary ferrite which are HAZ toughness inhibiting factors, thereby increasing the productivity and high heat input HAZ of thick steel plates. It has been found that toughness can be achieved at the same time. That is, by appropriately controlling the oxide composition, it is possible to ensure the production of intra-grain α, and conventionally, it is possible to suppress the formation of coarse Ti nitride that has been crystallized from the oxide in the molten steel, It has been found that, by appropriately controlling the steel material components, the formation of coarse grain boundary ferrite can be suppressed, so that a thick steel plate having excellent high heat input HAZ toughness can be obtained.

More specifically, among these oxides, oxides having an equivalent circle diameter of less than 2 μm are dispersed at 300 / mm ² or more, and oxides having an equivalent circle diameter of 2 μm or more are suppressed to 100 / mm ² or less. Thus, it was confirmed that excellent HAZ toughness was obtained.

  The present invention has been completed on the basis of the knowledge described above. The reasons for defining each constituent element are as follows.

(Constituent elements excluding oxygen are in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30. %, 1.5 ≦ REM / Zr and equivalent circle diameter of less than 2 μm is 300 oxides / mm ² or more)
By setting the equivalent circle diameter of the oxide to less than 2 μm, HAZ toughness can be promoted by promoting intragranular α. When the equivalent circle diameter of the oxide is 2 μm or more, the barrier energy when the coarse Ti nitride is crystallized decreases, and the amount of coarse Ti nitride generated increases. Further, the composition of the oxide is 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30 in mass%. %, 1.5 ≦ REM / Zr, a sufficient intragranular α formation cannot be obtained. In particular, when the REM / Zr ratio (% by mass) in the oxide is 1.5 or more, the amount of coarse crystallization Ti nitride generated on the surface of the oxide in the molten steel is reduced. In addition, when the number of oxides having an equivalent circle diameter of less than 2 μm is less than 300 pieces / mm ² , the starting point of intragranular α formation is insufficient, so the amount of intragranular α generation is also reduced, and sufficient HAZ toughness is obtained. Disappear.

(Equivalent circle diameter of 2 μm or more is 100 oxides / mm ² or less)
Of the oxides satisfying the above composition, an oxide having an equivalent circle diameter of 2 μm or more promotes brittle fracture and deteriorates HAZ toughness, so that it is preferably as small as possible. From this point of view, in the present invention, the oxide having an equivalent circle diameter of 2 μm or more is defined as 100 / mm ² or less.

(Ti nitride with equivalent circle diameter of 1 μm or more is 5 / mm ² or less)
When the number of Ti nitrides having an equivalent circle diameter of 1 μm or more exceeds 5 / mm ² , brittle fracture is promoted and HAZ toughness is deteriorated. In addition to having a rectangular parallelepiped shape, such a Ti nitride has a characteristic that the HAZ toughness is remarkably deteriorated due to stress concentration because the hardness is significantly higher than that of steel. Therefore, coarse Ti nitride needs to be controlled more strictly than coarse oxide.

(Production method)
The thick steel plate of the present invention that satisfies the above-mentioned requirements, that is, the constituent elements excluding oxygen are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% by mass. % <REM <50%, 2% <Zr <30%, and an oxide satisfying 1.5 ≦ REM / Zr, and among the oxides, 300 oxides having an equivalent circle diameter of less than 2 μm / mm ² or more, circle oxides of equivalent diameter above 2μm have 100 / mm ² or less, with present, there number of equivalent circle diameter of more than 1μm coarse Ti nitrides of Ti nitrides contained the In order to manufacture a steel plate of 5 pieces / mm ² or less, it is necessary to manufacture the steel plate so as to satisfy the following manufacturing requirements.

  The manufacturing requirement is that the amount of dissolved oxygen in molten steel is 0.002 to 0.01% by mass% by deoxidation using Mn, Si, etc. at the time of melting, and then the amount of REM added [REM] [REM] / [Zr], which is a mass ratio of Zr addition amount [Zr] to 1.8 or more, and from the addition of REM or Zr to Ca addition in the order of Al → Ti → (REM, Zr) → Ca. Each element may be added while controlling the time t1 to be 10 minutes or longer, and the cooling time t2 in the temperature range of 1500 to 1450 ° C. during casting may be within 300 seconds. Next, the reasons for defining these manufacturing requirements will be described in detail.

-The amount of dissolved oxygen in molten steel is 0.002 to 0.01% by deoxidation using Mn, Si, etc.
When the amount of dissolved oxygen is less than 0.002%, a required amount of oxide having an appropriate composition that becomes the starting point of intragranular α formation cannot be secured. On the other hand, if the amount of dissolved oxygen exceeds 0.01%, coarse oxides having an equivalent circle diameter of 2 μm or more increase, and the HAZ toughness is deteriorated.

-Mass ratio of REM addition amount [REM] and Zr addition amount [Zr]: [REM] / [Zr] is 1.8 or more-Time t1 from addition of REM or Zr to Ca addition is 10 minutes or more In the present invention The specified oxide has a feature that it has an action of promoting the formation of α within the grains and is difficult to function as a crystallization starting point of coarse Ti nitride. In particular, in order to set the REM / Zr ratio (mass%) in the oxide to 1.5 or more, [REM] / [Zr], which is the mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr]. It is necessary to make the REM or Zr oxide formation reaction sufficiently proceed prior to the addition of Ca, which is a strong deoxidizing element. Specifically, an oxide satisfying REM / Zr ≧ 1.5 having a predetermined number density can be obtained by controlling the time t1 from the addition of REM or Zr to the addition of Ca to 10 minutes or more. When the time t1 from the addition of REM or Zr to the addition of Ca is less than 10 minutes, the oxide satisfying REM / Zr ≧ 1.5 is insufficient.

  At the time of melting, the reason for adding Al → Ti → (REM, Zr) → Ca in this order is that if each element is added in an order other than this addition order, an appropriate composition that will be the starting point of intragranular α formation This is because the required number of oxides cannot be secured. In particular, since Ca is a strong deoxidizing element having a very strong deoxidizing power, if it is added prior to Ti or Al, oxygen associated with Ti or Al is remarkably reduced.

・ Cooling time t2 at 1500 to 1450 ° C. during casting is less than 300 seconds If cooling time t2 at 1500 to 1450 ° C. during casting exceeds 300 seconds, coarse Ti nitride crystallizes due to component segregation during solidification, and HAZ toughness Will deteriorate.

(Chemical composition)
Next, the chemical component composition in the thick steel plate of the present invention will be described. The steel plate of the present invention has characteristics of the base material (thick steel plate) if the content of each chemical component (element) is not within the proper range even if the oxide dispersion state described above is appropriate. And HAZ cannot be improved. Therefore, in the thick steel plate of the present invention, it is also a requirement that the content of each chemical component is within the range described below. Among these chemical components, the content of Al, Ca, Ti, etc. constituting the oxide includes the amount constituting the oxide, as is apparent from its action and effect. In addition, all the content (%) of the following chemical component shows the mass%.

C: 0.03-0.12%
C is an essential element for ensuring the strength of the steel sheet. If the C content is lower than 0.03%, the required strength cannot be ensured. On the other hand, when the C content is excessive, a large amount of hard island martensite (MA) is generated, leading to deterioration of the toughness of the base material. Therefore, the C content needs to be 0.12% or less. The minimum with preferable content of C is 0.04%, and a preferable upper limit is 0.10%.

Si: 0.25% or less (including 0%)
Si is not an essential element, but is an element useful for securing strength by solid solution strengthening. However, if it is added excessively, in HAZ, hard island martensite (MA) increases, leading to degradation of HAZ toughness. Therefore, the upper limit of the Si content is 0.25%. Moreover, a preferable upper limit is 0.21% and a more preferable upper limit is 0.18%.

Mn: 1.0-2.0%
Mn is an element useful for ensuring the strength of the steel sheet. In order to exhibit such an effect effectively, it is necessary to contain 1.0% or more. However, if the content exceeds 2.0% excessively, the strength of the HAZ increases excessively and the toughness deteriorates, so the Mn content is set to 2.0% or less. The preferable lower limit of the Mn content is 1.4%, and the preferable upper limit is 1.8%.

P: 0.03% or less (excluding 0%)
Since P is an impurity element that easily causes grain boundary fracture and adversely affects toughness, its content is preferably as small as possible. From the viewpoint of securing HAZ toughness, the P content needs to be suppressed to 0.03% or less, and preferably 0.02% or less. However, it is difficult to make P in steel 0% industrially.

S: 0.015% or less (excluding 0%)
Since S is an element that degrades the HAZ toughness by forming Mn sulfide at the prior austenite grain boundaries in the HAZ, its content is preferably as small as possible. From the viewpoint of ensuring HAZ toughness, the S content must be suppressed to 0.015% or less, and preferably 0.010% or less. However, it is difficult to industrially make S in steel 0%.

Al: 0.004 to 0.05%
Al is an element that forms an oxide serving as a starting point of the intra-grain α. When the content is less than 0.004%, a predetermined oxide form cannot be obtained and coarse Ti nitride is crystallized. On the other hand, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.05% or less. The preferable lower limit of the Al content is 0.007%, and the preferable upper limit is 0.04%.

Ti: 0.010 to 0.050%
By adding Ti prior to REM, Zr, and Ca, it becomes possible to finely disperse the oxide having the action of promoting the formation of intra-grain α. In order to exhibit such an effect effectively, it is necessary to contain Ti 0.010% or more. However, if the content is excessive, a large amount of coarse Ti nitride is crystallized and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.050% or less. The preferable lower limit of the Ti content is 0.012%, the preferable upper limit is 0.035%, and the more preferable upper limit is 0.025%.

REM: 0.0003-0.02%, Zr: 0.0003-0.02%
REM (rare earth element) and Zr are added prior to the addition of Ca after the addition of Ti, thereby forming an oxide effective for the generation of intragranular α. Such effects increase as their content increases, but in order to effectively exhibit these effects, it is necessary to contain 0.0003% or more of all. However, if these are excessively contained, the oxide becomes coarse and deteriorates the HAZ toughness, so both should be suppressed to 0.02% or less. The more preferable lower limit of these contents is 0.0005%, and the more preferable upper limit is 0.015%.

Ca: 0.0005 to 0.010%
When Ca is added 10 minutes or more after the addition of Ti, REM, and Zr, it forms an oxide that is effective in generating intragranular α and suppresses crystallization of coarse Ti nitride. In order to exhibit such an effect effectively, it is necessary to contain 0.0005% or more. However, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.010% or less. The preferable lower limit of the Ca content is 0.0008%, and the preferable upper limit is 0.008%.

N: 0.002 to 0.010%
N is an element useful for securing the toughness of HAZ by forming fine Ti nitride. By making the content 0.002% or more, desired Ti nitride can be secured. However, if the content is excessive, crystallization of coarse Ti nitride is promoted, so it is necessary to suppress it to 0.010% or less. The preferable lower limit of the N content is 0.003%, and the preferable upper limit is 0.008%.

B: 0.0005 to 0.005%
Even if intragranular α is generated and crystallization of coarse Ti nitride is suppressed, if coarse grain boundary ferrite is produced in HAZ, the obtained HAZ toughness is limited. B has the effect of improving the HAZ toughness by suppressing the formation of coarse grain boundary ferrite. The effect increases as the content increases, but in order to effectively exhibit such an effect, it is necessary to contain 0.0005% or more. However, when the content is excessive, coarse bainite buckets from the prior austenite grain boundaries are promoted, and the HAZ toughness is deteriorated. Therefore, it is preferably suppressed to 0.005% or less. A preferable lower limit of the B content is 0.0010%, a more preferable lower limit is 0.0015%, and a preferable upper limit is 0.004%.

  The above are the essential elements specified in the present invention, and the balance is iron and inevitable impurities. As an inevitable impurity, mixing of elements such as Sn, As, and Pb brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. Further, it is also effective to positively contain the following elements, and the characteristics of the thick steel plate are further improved depending on the kind of chemical components (elements) contained.

Selected from the group consisting of Ni: 0.05 to 1.50%, Cu :: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50%. One or more types Ni, Cu, Cr, and Mo are all effective elements for increasing the strength of the steel sheet, and the effect thereof increases as the content thereof increases. In order to exhibit such an effect effectively, it is preferable to contain 0.05% or more of all. However, if they are contained excessively, the strength is excessively increased, and the HAZ toughness is deteriorated. The more preferable lower limit of the content thereof is 0.10%, and the more preferable upper limit is 1.20%.

Nb: 0.002-0.10% and / or V: 0.002-0.10%
Nb and V precipitate as carbonitrides and are effective elements for improving the base material toughness by suppressing the coarsening of γ grains. Although the effect increases as the content thereof increases, in order to effectively exhibit such an effect, it is preferable that the content is 0.002% or more. However, if they are contained excessively, the HAZ structure is coarsened and the HAZ toughness is deteriorated. The more preferable lower limit of the content thereof is 0.005%, and the more preferable upper limit is 0.08%.

(Parameter)
After satisfying the above chemical composition, the thick steel plate of the present invention has an A value determined from the formula A = 10 ⁴ × [B] × (0.4 + 30 × [Ti] −82 × [N]). 5.0 ≦ A ≦ 25.0 must be satisfied (however, in the above formula, [] indicates the content (% by mass) of each element). In order to suppress the formation of coarse grain boundary ferrite, B must be added and free B must be ensured. The A value obtained from the above equation is for controlling the amount of free B. When this A value is less than 5.0, the formation of coarse grain boundary ferrite cannot be suppressed, while the A value is 25.0. When exceeding, the coarse bainite bucket from an austenite grain boundary will be accelerated | stimulated and HAZ toughness will fall.

  In the above equation for obtaining the A value, the coefficient of each element and the control range were obtained experimentally based on the following idea.

Since B exists mainly in the steel as free B and B nitride, the amount of free B is substantially determined by the amount of B added and the amount of B nitride. B nitride is produced by free N generated by dissolution of Ti nitride during high-temperature heating of HAZ combined with B in the subsequent cooling process. Therefore, in deriving the above equation, a linear regression equation of [Ti] and [N] representing the free N amount at the time of HAZ high temperature heating (1400 ° C.) is calculated by calculation using thermodynamic calculation software Thermo-Calc. Asked. Further, the amount of B nitride generated during the HAZ cooling was considered to be proportional to the product of the amount of free N and the amount of added B, and the proportionality coefficient was determined by comparing with the amount of free B obtained experimentally. The formula representing the amount of B nitride calculated in this way is subtracted from the amount of added B is the formula A = 10 ⁴ × [B] × (0.4 + 30 × [Ti] −82 × [N]. .

  In addition, in the description of the chemical component composition, it has been explained that it is effective to contain one or more selected from the group consisting of Ni, Cu, Cr, and Mo. In that case, their content (mass%) However, it is necessary to satisfy [Ni] + [Cu] + [Cr] + [Mo] <2.5%.

  Coarse Ti nitride crystallizes in a liquid phase in which Ti and N are concentrated by solidification segregation in the solidification stage of the molten steel. When [Ni] + [Cu] + [Cr] + [Mo] exceeds 2.5%, the solidification temperature is lowered, and the liquid phase remains until a low temperature at which the driving force of coarse Ti nitride crystallization becomes large. Therefore, the amount of coarse Ti nitride produced increases.

  Although this invention is invention regarding a thick steel plate, generally a thick steel plate shows the steel plate whose plate | board thickness is 3.0 mm or more as defined by JIS. On the other hand, the thick steel plate of the present invention was invented for welding thick steel plates having a thickness of 50 mm or more, and the target steel plate can be said to be a steel plate having a thickness of 50 mm or more. However, these are merely preferred embodiments and do not exclude application of the present invention to thick steel plates having a thickness of less than 50 mm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In the examples of the present invention, first, steels having respective component compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace (VIF: 150 kg), and then cast into slabs (cross-sectional shape: 150 mm × 250 mm) was cast, and hot rolling was further performed using the slab, thereby obtaining a hot rolled sheet having a thickness of 80 mm. The hot rolling conditions were heating before rolling: 1100 ° C. × 3 hours, finish rolling temperature: 780 ° C. or higher, average cooling rate up to 450 ° C .: 6 ° C./s, and cooling stop temperature: 450 ° C.

  Tables 3 and 4 show the controlled conditions in producing this hot-rolled sheet (thick steel sheet). The conditions are as follows: dissolved oxygen amount [Of] (% by mass) in molten steel before addition of Al (Ti), addition order of Al, Ti, REM, Zr, Ca, time t1 from addition of REM or Zr to addition of Ca, Mass ratio of REM addition amount [REM] and Zr addition amount [Zr]: [REM] / [Zr] (described in the table as REM / Zr), cooling time t2 at 1500 to 1450 ° C. during casting.

  In Tables 1 and 2, REM was added in the form of a misch metal containing, by mass%, about 50% Ce and about 25% La. In Tables 1 and 2, “-” indicates that the corresponding element is not added.

  In Tables 3 and 4, the order of addition of Al, Ti, REM, Zr, and Ca is “◯” when Al → Ti → (REM, Zr) → Ca, and other orders. Indicated by “x”.

  Using each hot-rolled sheet (thick steel plate) manufactured according to the above requirements, the number density N1 of oxides having an equivalent circle diameter of less than 2 μm, the number density N2 of oxides having an equivalent circle diameter of 2 μm or more, and the equivalent circle diameter The number density N3 and the HAZ toughness of Ti nitride having a thickness of 1 μm or more were determined by the following measurements. These measurement results are shown in Tables 5 and 6.

(Measurement of number density of oxides with equivalent circle diameter less than 2 μm)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. The parallel cross section was observed using a field emission scanning electron microscope “SUPRA35 (trade name)” (hereinafter referred to as FE-SEM) manufactured by Carl Zeiss. The observation conditions were as follows: magnification: 5000 times, observation visual field: 0.0024 mm ² , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. In addition, it was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 seconds. Then, the number (N1) of oxides having an equivalent circle diameter of less than 2 μm was determined by converting to a number density equivalent to 1 mm ² . However, oxides having an equivalent circle diameter of 0.2 μm or less were excluded from the analysis because the reliability of EDX was not sufficient.

(Measurement of number density of oxides with equivalent circle diameter of 2 μm or more)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. Parallel cross sections were observed using FE-SEM. The observation conditions were as follows: magnification: 1000 times, observation visual field: 0.06 mm ² , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. In addition, it was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 seconds. Then, the number (N2) of oxides having an equivalent circle diameter of 2 μm or more was determined by converting into a number density equivalent to 1 mm ² .

(Measurement of number density of Ti nitride with equivalent circle diameter of 1 μm or more)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. A parallel section was photographed with 20 optical fields at a magnification of 200 times using an optical microscope, and the number of coarse Ti nitrides was counted and converted to a number density corresponding to 1 mm ² . Area of the measurement image is one field per 0.148 mm ^2, per sample 2.96 mm ^2. Ti nitrides were identified based on shape and color, and square-shaped and bright orange inclusions were considered Ti nitrides. The equivalent circle diameter of Ti nitride was calculated by analysis software. Coarse Ti nitride is often crystallized starting from an oxide. In that case, the internal oxide was excluded from the measurement of the equivalent circle diameter.

(Evaluation of HAZ toughness)
Test specimens for welded joints were collected from each thick steel plate, subjected to V pre-processing, and then subjected to electrogas arc welding at a heat input of 50 kJ / mm. From these test pieces, Charpy impact test pieces (V notches of JIS Z 2242) in which notches were formed in the HAZ near the weld line (bond) at a depth t / 4 (t: thickness) from the surface of each thick steel plate. Three test specimens) were collected and subjected to a Charpy impact test at −40 ° C., the absorbed energy (vE ₋₄₀ ) was measured, and the average value and the minimum value thereof were obtained. From this measurement result, an average value of vE- ₄₀ exceeding 180 J and a minimum value exceeding 120 J were evaluated as excellent in HAZ toughness.

In addition, the Charpy impact test was performed under the same conditions as described above except that electrogas arc welding was performed at a heat input of 60 kJ / mm, and the absorbed energy (vE- ₄₀ ) of three test pieces was measured. The average value was obtained. From this measurement result, those having an average value of vE- ₄₀ exceeding 120 J were evaluated as having excellent HAZ toughness.

  No. 1 to 30 are examples of the invention that satisfy the requirements of the present invention, in which chemical composition, oxide, Ti nitride dispersion, etc. are appropriately made, and the HAZ toughness when the heat input is 50 kJ / mm ( It can be seen that the HAZ toughness (average value) is excellent when the average value and the minimum value) and the heat input amount are 60 kJ / mm. That is, no. It can be said that 1-30 are thick steel plates excellent in the toughness of the weld heat affected zone.

  In contrast, no. 31 to 52 are comparative examples not satisfying any of the requirements of the present invention, and HAZ toughness (average value and minimum value) when the heat input is 50 kJ / mm, and the heat input is 60 kJ / mm. It can be seen that any of the HAZ toughness (average value) does not satisfy the evaluation criteria.

Claims (3)

In mass%, C: 0.03-0.12%, Si: 0.25% or less (including 0%), Mn: 1.0-2.0%, P: 0.03% or less (0% S: 0.015% or less (excluding 0%), Al: 0.004-0.05%, Ti: 0.010-0.050%, REM: 0.0003-0. 02%, Zr: 0.0003-0.02%, Ca: 0.0005-0.010%, N: 0.002-0.010%, B: 0.0005-0.0050%, A steel plate with the balance being iron and inevitable impurities,
The A value obtained from the formula A = 10 ⁴ × [B] × (0.4 + 30 × [Ti] −82 × [N]) satisfies 5.0 ≦ A ≦ 25.0,
Further, the constituent elements excluding oxygen are in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <. 30%, containing oxides satisfying 1.5 ≦ REM / Zr, and among the oxides, 300 / mm ² or more of oxides having an equivalent circle diameter of less than 2 μm and an equivalent circle diameter of 2 μm or more 100 oxides / mm ² or less are present,
A steel plate excellent in toughness of the weld heat affected zone, characterized in that, among the contained Ti nitrides, the number of Ti nitrides having an equivalent circle diameter of 1 μm or more is 5 pieces / mm ² or less.
However, in the above formula, [] indicates the content (% by mass) of each element. Furthermore, in terms of mass%, Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50% Containing one or more selected from the group consisting of:
The thick steel plate excellent in toughness of the heat affected zone according to claim 1, wherein [Ni] + [Cu] + [Cr] + [Mo] <2.5% is satisfied.
However, in the above formula, [] indicates the content (% by mass) of each element.   The toughness of the heat affected zone according to claim 1 or 2, further comprising Nb: 0.002 to 0.10% and / or V: 0.002 to 0.10% in mass%. Excellent steel plate.