JP5883369B2 - Thick steel plate with excellent toughness in weld heat affected zone - Google Patents

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 the welding heat input is increased, and special measures are required to obtain stable HAZ toughness.

  Inventors have also proposed a technique for improving the high heat input HAZ toughness by precisely controlling the size and number of fine Ti nitrides in Patent Document 3. However, the assumed heat input is only 55 kJ / mm, and further improvement is necessary to cope with further increase in welding heat input.

  Patent Documents 4 to 7 propose a technique of using oxide inclusions that are stable at high temperatures 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, although Patent Document 4 describes that good HAZ characteristics can be obtained by the presence of an oxide containing REM or Zr, the assumed heat input is only at a low level, and it is not necessarily large input. No good HAZ properties can be obtained by thermal welding. Patent Document 5 describes a technique that uses an oxide containing REM or Zr as in Patent Document 4, 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 6 describes a technique for obtaining high HAZ toughness by using both oxide inclusions and Ti-containing inclusions as γ grain growth pinning particles. However, in Patent Document 6, the HAZ toughness is evaluated by a thermal cycle test that simulates high heat input welding, but the maximum heating temperature is 1400 ° C. and a temperature at which some Ti-containing nitride remains. ing. However, the highest heating temperature of the HAZ is partly higher than 1450 ° C., and the disappearance of the Ti-containing nitride is further promoted. Therefore, in order to accurately evaluate the high heat input HAZ toughness, it is desirable to actually perform a high heat input welding test. In addition, the inventors have proposed a technique that utilizes the gamma grain growth pinning effect of fine oxide inclusions in Patent Document 7, 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 8 and MnS is proposed, Inventors have proposed the technique which accelerates | stimulates intragranular alpha production | generation by controlling the inclusion shape by patent document 9. 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 have proposed it as Patent Document 10. 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 8, the assumed heat input itself is small, and even in the techniques described in Patent Document 9 and Patent Document 10, 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 11 and Patent Document 12, a technique capable of obtaining 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 11, the amount of Ca added is controlled based on the amount of dissolved oxygen before the addition of Ca 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 12, since it is necessary to hold for 40 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 JP 2010-95781 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, wherein, in mass%, C: 0.03~0.12%, Si : 0.10~0.25%, Mn: 1.00 ~ 2.00%, P: 0.030 % Or less (excluding 0%), S: 0.015% or less (not including 0%), Al: 0.004 to 0.050 %, Ti: 0.010 to 0.050 %, REM: 0 .0003~ 0.0200%, Zr: 0.0003~ 0.0200 %, Ca: 0.0005~ 0.0100%, N: 0.0020 contains ~ 0.0100%, the balance being iron and unavoidable Constituent elements excluding oxygen, which are impurities, are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <% by mass. Contains an oxide satisfying Zr <30%, 1.0 ≦ REM / Zr, and is equivalent to a circle of the oxides There 0.2μm oxides and 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 Ti nitride having a diameter of 1 μm or more is 7 pieces / mm ² or less, Ti nitride having a circle equivalent diameter of 20 nm or more is 1.0 × 10 ⁶ pieces / mm ² or more, and the equivalent circle diameter is 20 nm. The size of the above-mentioned Ti nitride is divided into regions every 5 nm in ascending order of the equivalent circle diameter from 20 nm, and the range of equivalent circle diameter for each region is set to (di-5) or more and less than di. When the di in the region where the number density of Ti nitrides existing in the region is the largest is df, the average equivalent circle diameter da of the Ti nitride having an equivalent circle diameter of 20 nm to less than 500 nm is | The relationship da−df | /da≦0.35 A thick steel sheet satisfying, from the steel plate, it was taken test pieces for welded joints, after performing V destination processing, heat input: the electro-gas welding performed at 50 kJ / mm, the depth from the surface of Three Charpy impact test pieces with notches formed in the HAZ near the weld line at the t / 4 position were collected and the absorbed energy when the Charpy impact test was conducted at -40 ° C: measured value of vE- ₄₀ The average value exceeds 180 J, the minimum value exceeds 120 J, and specimens for welded joints are collected from the thick steel plate, subjected to V pre-processing, and then subjected to electrogas welding at a heat input of 60 kJ / mm. Then, three Charpy impact test pieces in which notches were machined in the HAZ near the weld line at a depth t / 4 from the surface were collected, and the absorbed energy when the Charpy impact test was performed at −40 ° C .: flat of the measured values of vE _-40 Value is an excellent thick steel sheet toughness of the heat affected zone, characterized in that more than 120 J. However, the above-mentioned t is the plate thickness, and the Charpy impact test piece is a V-notch test piece of JIS Z 2242.

  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).

According to the second aspect of the present invention, the constituent elements excluding oxygen are, in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <. 50%, 2% <Zr < 30%, meets the 1.5 ≦ REM / Zr, and a feature that the circle equivalent diameter oxides of less than 0.2μm ultra 2μm is present 300 / mm ² or more It is a thick steel plate excellent in the toughness of the welding heat affected zone of Claim 1.

  The invention according to claim 3 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 is a thick steel plate excellent in the toughness of the welding heat affected zone of Claim 1 or 2 characterized by containing 1 or more types chosen from the group which consists of 0.05 to 1.50%.

The invention according to claim 4 further contains Nb: 0.002 to 0.100 % and / or V: 0.002 to 0.100 % by mass%. It is a thick steel plate excellent in the toughness of the welding heat affected zone in any one of.

  The invention according to claim 5 further comprises B: 0.0005 to 0.0050%, and the thick steel plate having excellent toughness of the weld heat affected zone according to any one of claims 1 to 4 It is.

  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 produce thick steel sheets by ensuring the formation of intra-granular α at the oxide origin, suppressing the formation of coarse Ti nitride, which is a HAZ toughness inhibiting factor, and appropriately controlling the Ti nitride dispersion form. And high heat input HAZ toughness were found to be compatible. That is, by appropriately controlling the oxide composition, it is possible to ensure the formation of α within the grains, and also to appropriately control the size and number of Ti nitrides and suppress the coarsening of the old γ grains. It has been found that a thick steel plate having excellent high heat input HAZ toughness can be obtained because the grain boundary ferrite generated at the grain boundary can be refined.

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. As a result, 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. %, 1.0 ≦ REM / Zr and the equivalent circle diameter is less than 2 μm, 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%. %, 1.0 ≦ REM / Zr, a sufficient intragranular α formation cannot be obtained. By setting the REM / Zr ratio (mass%) in the oxide to 1.5 or more, the amount of coarse crystallization Ti nitride formed on the surface of the oxide in the molten steel is further reduced, and the HAZ toughness is further improved. Is realized.

(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.

  In the present invention, the form of Ti nitride is also defined in detail. Ti nitride contributes to HAZ toughness improvement by suppressing the coarsening of γ grains during high-temperature heating of HAZ and reducing the size of grain boundary ferrite generated during cooling. In order to sufficiently suppress the coarsening of γ grains, it is of course necessary to disperse a large number of Ti nitride particles, but the present inventors have further increased the HAZ high-temperature heating as the size of the Ti nitride particles becomes more uniform. That the dissolution rate of Ti nitride decreases at the same time, and by controlling the size and number of Ti nitrides appropriately, it is clear that the effect of suppressing the coarsening of γ grains can be obtained even with high heat input welding I made it. Specifically, high large heat input HAZ toughness is realized by satisfying the following two conditions.

(Ti nitride with equivalent circle diameter of 1 μm or more is 7 pieces / mm ² or less)
When the number of Ti nitrides having an equivalent circle diameter of 1 μm or more exceeds 7 / 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.

(Ti nitride with an equivalent circle diameter of 20 nm or more is 1.0 × 10 ⁶ pieces / mm ² or more)
When the number of Ti nitrides having an equivalent circle diameter of 20 nm or more is less than 1.0 × 10 ⁶ pieces / mm ² , Ti nitride particles necessary for suppressing γ grain coarsening cannot be secured. The ultrafine Ti nitride particles having an equivalent circle diameter of less than 20 nm disappear in a short time during high-temperature heating during large heat input welding, and hardly contribute to the suppression of γ grain coarsening.

(| Da-df | /da≦0.35)
The smaller the size of Ti nitride particles, the more unstable the energy, and more specifically, the smaller the average size of all the particles, the easier it is to disappear during HAZ high-temperature heating. Therefore, even if it is larger than the average size or smaller than the average size, as the number of Ti nitride particles having a size relatively close to the average size increases, the number of substantial particles contributing to the suppression of γ grain coarsening increases.

  In the present invention, in the Ti-nitride size-number histogram, the substantial number of Ti nitride particles is controlled by controlling the difference between the recorded size and the average size of the largest number of Ti nitrides. As a result, it was found that a high γ grain coarsening suppressing effect was realized.

  More specifically, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region (di-5). ) And less than di, and the average of Ti nitride having an equivalent circle diameter of less than 500 nm when the di of the region where the number density of Ti nitride existing in each region is the largest is df The difference from the equivalent circle diameter da is controlled to be small. By this control, the substantial number of Ti nitride particles is increased, and a high γ grain coarsening suppressing effect is realized.

  In calculating the average equivalent circular diameter of Ti nitride, transmission electron microscope (TEM) observation is performed under the conditions described later in the example section, and each Ti nitride in the observation field is analyzed by image analysis. After measuring the area of the product and calculating the equivalent circle diameter of each Ti nitride from the area, the arithmetic average of the equivalent circle diameter was obtained for the Ti nitride having an equivalent circle diameter of 20 nm or more and less than 500 nm.

  Specifically, if the value obtained from | da−df | / da exceeds 0.35, even if the number of Ti nitride particles is large, γ grain coarsening is not sufficiently suppressed, and the high Heat input HAZ toughness cannot be obtained.

(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%, 1.0 ≦ REM / Zr satisfying oxide, 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, among Ti nitrides contained, Ti nitride circle equivalent diameter of more than 1μm is 7 pieces / mm ^2, circles Ti nitrides of equivalent diameter than 20nm is 1.0 × 10 ⁶ cells / mm ² or more, there, further, | da-df | that /da≦0.35 a steel plate satisfying the relation In order to manufacture, it is necessary to manufacture thick steel plates that satisfy the following manufacturing requirements. The

  The manufacturing requirement is that at the time of melting, the amount of dissolved oxygen in the molten steel is 0.002 to 0.01% by mass% by deoxidation using Mn, Si, etc., then Al → Ti → (REM , Zr) → Ca in the order of REM or Zr to Ca addition, while controlling the time t1 to be 5 minutes or more, each element was added, and the temperature at 1500 to 1450 ° C. at the time of casting The cooling time t2 in the range may be set within 300 seconds, and the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting may be set within 680 seconds. Further, by setting [REM] / [Zr], which is a mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr], to 1.8 or more and t1 to 10 minutes or more, a more appropriate oxide form Is realized, and the coarse crystallized Ti nitride formed on the surface of the oxide in the molten steel is reduced, so that more excellent HAZ toughness can be obtained. 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.

The time t1 from the addition of REM or Zr to the addition of Ca is 5 minutes or more. The oxide defined in the present invention has an action of promoting the formation of intra-granular α and is difficult to function as a crystallization starting point of coarse Ti nitride. Has characteristics. In particular, in order to set the REM / Zr ratio (mass%) in the oxide to 1.0 or more, the REM or Zr oxide formation reaction is sufficiently advanced prior to the addition of Ca, which is a strong deoxidizing element. There is a need. Specifically, an oxide satisfying REM / Zr ≧ 1.0 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 5 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 ≧ 1.0 is insufficient. In addition to this, by setting [REM] / [Zr], which is a mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr], to 1.8 or more and t1 to 10 minutes or more, REM / An oxide satisfying Zr ≧ 1.5 can be obtained at a predetermined number density.

  At the time of melting, the reason for adding Al → Ti → (REM, Zr) → Ca in this order is that when each element is added in an order other than this addition order, an appropriate composition that becomes the starting point of intragranular α formation is obtained. 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 1500-1450 ° C during casting is within 300 seconds When cooling time t2 at 1500-1450 ° C during casting exceeds 300 seconds, coarse oxides increase. Alternatively, coarse Ti nitrides crystallize due to component segregation during solidification, and HAZ toughness deteriorates.

The cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting is within 680 seconds. When the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting exceeds 680 seconds, | da−df | / da ≦ The relational expression of 0.35 cannot be satisfied. The cause can be considered as follows.

  Ti nitrides produced during casting include A.I. Ti nitride crystallizing in molten steel; Ti nitride formed in a solidified segregation part of solidified steel, C.I. There is Ti nitride generated in the non-solidified segregation part of the solidified steel, which is generated in the order of A → B → C, and the size (particle diameter) is in the order of A> B> C. On the other hand, the number of particles is in the order of A <B <C, and most of the Ti nitride having a size corresponding to the df is C Ti nitride. Further, since the Ti nitride of A has a smaller number of particles than the Ti nitrides of B and C, the average equivalent circular diameter da of the Ti nitride is hardly affected. Therefore, it can be said that it is necessary to control the formation of B and C Ti nitrides in order to keep | da-df | / da within a predetermined range. When the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting exceeds 680 seconds, B Ti nitride grows prior to the formation of C Ti nitride, so the average equivalent circle diameter of Ti nitride It is considered that da becomes large and | da−df | / da exceeds 0.35.

(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: 0.10 to 0.25%
Si is an element that improves the activity of Ti, and needs to be added appropriately in order to realize a predetermined Ti nitride form. If the added amount is less than 0.10%, the number density of Ti nitride having an equivalent circle diameter of 20 nm or more cannot be secured at 1.0 × 10 ⁶ pieces / mm ² . On the other hand, if it exceeds 0.25%, coarse Ti nitride is likely to be formed, and a hard MA structure is formed, and a predetermined HAZ toughness cannot be obtained. A preferred lower limit is 0.12%, a more preferred lower limit is 0.14%, a preferred upper limit is 0.22%, and a more preferred upper limit is 0.20%.

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 the intragranular transformation is not sufficiently promoted, so that the HAZ toughness is deteriorated. 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%
Ti is an element that forms Ti nitride, and by adding it prior to REM, Zr, and Ca, it is possible to finely disperse an oxide having an action of promoting the formation of intragranular α. In order to realize a predetermined Ti nitride and oxide form, it is necessary to contain 0.010% or more of Ti. 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. These oxides become a more preferable intra-granular α-generation site by complex precipitation of Ti nitride. 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%.

  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%.

B: 0.0005 to 0.0050%
B has the effect of improving the HAZ toughness by suppressing the formation of coarse grain boundaries α. Such an effect increases as the content increases, but in order to exert the effect effectively, the content is preferably 0.0005% or more. More preferably, it is 0.0010% or more, More preferably, it is 0.0015% or more. However, when the B content is excessive, coarse bainite buckets from the prior austenite grain boundaries are promoted, and the HAZ toughness is reduced. A preferable upper limit is 0.0050%, a more preferable upper limit is 0.0040%, and a still more preferable upper limit is 0.0035%.

  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 preferable that [Ni] + [Cu] + [Cr] + [Mo] <2.5% is satisfied (however, [] in the preceding formula indicates the content (% by mass) of each element). .

  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.

(| Da-df | /da≦0.35)
| Da-df | / da is a parameter related to the number of Ti nitrides contributing to the suppression of γ grain coarsening during high-temperature heating of the HAZ. If it exceeds 0.35, the coarsening of the γ grains is not sufficiently suppressed, and the predetermined HAZ toughness cannot be ensured. A preferable upper limit is 0.30, and a more preferable upper limit is 0.25.

  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 as REM / Zr in the table), cooling time t2 at 1500 to 1450 ° C. during casting, This is the cooling time t3 at 1300 to 1200 ° C.

  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 plate (thick steel plate) manufactured according to the above requirements, the equivalent circle diameter is less than 2 μm, and Ti, Al, Ca, REM, and Zr are contained in a predetermined concentration range, and [REM] / [Zr ] The number density N1 of the oxide satisfying ≧ 1.0, the equivalent circle diameter is less than 2 μm, and Ti, Al, Ca, REM, and Zr are contained in a predetermined concentration range, and [REM] / [Zr] ≧ 1.5 The number density NA of oxides, the number density N2 of oxides with an equivalent circle diameter of 2 μm or more, the number density N3 of Ti nitrides with an equivalent circle diameter of 1 μm or more, and the number of Ti nitrides with an equivalent circle diameter of 20 nm or more Density N4, | da-df | / da, and HAZ toughness 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 area: 0.048 mm ² . 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 of oxides (N1, NA) having an equivalent circle diameter of less than 2 μm was determined by converting into 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 cross section was photographed with 20 optical fields using an optical microscope at a magnification of 200 ×, and the number of coarse Ti nitrides was counted and calculated by converting the number density (N3) 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.

(Measurement of number density of Ti nitride having equivalent circle diameter of 20 nm or more and calculation of | da-df | / da)
A test piece is cut out from the t / 4 position of each thick steel plate, a replica TEM test piece is produced from a cross section parallel to the rolling direction and the plate thickness direction, and observed with a transmission electron microscope (TEM) at an observation magnification of 15000 times and an observation field of view 6 After observing four visual fields under the condition of .84 μm × 8.05 μm, particles containing Ti and N were discriminated by EDX (energy dispersive X-ray fluorescence analyzer) to obtain a Ti-containing nitride. Additionally the image analysis, the area of the Ti-containing nitride in the visual field was measured, in terms of circle-equivalent diameter is measured the number of the above Ti-containing nitride 20 nm, number density in terms of per 1 mm ² (N4 ) In addition, from the obtained data, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region. (Di-5) or more and less than di, the df when the di of the region where the number density of Ti nitride existing in each region is the most is df, and the equivalent circle diameter is 20 nm or more and less than 500 nm The average equivalent circle diameter da of Ti nitride was obtained, and | da-df | / da was calculated.

(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. Moreover, it evaluated that the average value of vE- ₄₀ exceeds 150J especially excellent in HAZ toughness.

  No. 1 to 35 are examples of the invention that satisfy the requirements of the present invention, in which chemical composition, oxide, Ti nitride dispersion, etc. are appropriately performed, 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 1-35 are thick steel plates excellent in the toughness of the weld heat affected zone.

Furthermore, the thick steel plate that satisfies the requirements of claim 2 can be evaluated as a thick steel plate having an average value of vE- ₄₀ of more than 150 J and particularly excellent in toughness of the weld heat affected zone.

  In contrast, no. 36 to 55 are comparative examples that do not satisfy any of the requirements of the present invention, and the 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 (5)

In mass%, C: 0.03 to 0.12%, Si: 0.10 to 0.25%, Mn: 1.00 to 2.00 %, P: 0.030 % or less (excluding 0%) ), S: 0.015% or less (excluding 0%), Al: 0.004 to 0.050 %, Ti: 0.010 to 0.050 %, REM: 0.0003 to 0.0200 %, Zr: 0.0003~ 0.0200%, Ca: 0.0005~ 0.0100%, N: 0.0020 contains ~ 0.0100%, balance being iron and unavoidable impurities,
Constituent elements excluding oxygen are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30% in mass%. contains an oxide satisfying 1.0 ≦ REM / Zr, and, among the oxide, circles oxide equivalent diameter of less than 0.2μm ultra 2μm 300 pieces / mm ² or more, the equivalent circle diameter 2μm There are 100 or more oxides / mm ² or more of the above oxides,
Among Ti nitrides contained, Ti nitride with an equivalent circle diameter of 1 μm or more is 7 pieces / mm ² or less, Ti nitride with an equivalent circle diameter of 20 nm or more is 1.0 × 10 ⁶ pieces / mm ² or more, Exists,
Further, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region (di-5). The df when the di is df in the region where the number density of Ti nitrides present in each region is the largest, and less than di,
The average equivalent circle diameter da of the Ti nitride having an equivalent circle diameter of 20 nm or more and less than 500 nm is:
A thick steel plate satisfying the relational expression | da-df | /da≦0.35 ,
A specimen for a welded joint is collected from the thick steel plate, subjected to V-preprocessing, and then subjected to electrogas welding at a heat input of 50 kJ / mm, and a weld line at a depth t / 4 from the surface. Three samples of Charpy impact test pieces in which notches were machined in the nearby HAZ were collected , and the average value of the measured value of absorbed energy: vE- ₄₀ when the Charpy impact test was performed at −40 ° C. exceeded 180 J. Minimum value exceeds 120J,
A specimen for a welded joint is collected from the thick steel plate and subjected to V-tip machining, and then electrogas welding is performed at a heat input of 60 kJ / mm, and a weld line at a depth t / 4 from the surface. Three Charpy impact test pieces with notches formed in nearby HAZ were sampled and the average value of measured values of absorbed energy: vE- ₄₀ exceeded 120 J when Charpy impact test was conducted at -40 ° C. A thick steel plate with excellent toughness in the heat affected zone.
However, the above-mentioned t is the plate thickness, and the Charpy impact test piece is a V-notch test piece of JIS Z 2242. Further, the constituent elements excluding oxygen are in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <. 30%, to meet the 1.5 ≦ REM / Zr, and equivalent circle diameter of oxides of less than 0.2μm ultra 2μm is characterized by the presence 300 / mm ² or more according to claim 1 welding heat according A thick steel plate with excellent toughness in the affected area.   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% The thick steel plate excellent in toughness of the weld heat affected zone according to claim 1, comprising at least one selected from the group consisting of: The welding heat according to any one of claims 1 to 3, further comprising Nb: 0.002 to 0.100 % and / or V: 0.002 to 0.100 % in mass%. A thick steel plate with excellent toughness in the affected area.   Furthermore, B: 0.0005 to 0.0050% is contained, The thick steel plate excellent in the toughness of the welding heat affected zone in any one of the Claims 1 thru | or 4 characterized by the above-mentioned.