JP5320274B2 - Thick steel plate with excellent toughness and strength uniformity in the heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate which can enhance not only an average value of toughness in HAZ but also the minimum value even when high-heat input welding has been conducted and also has a superior uniformity of strength properties in the sheet thickness direction. <P>SOLUTION: This thick steel plate satisfies a predetermined chemical component composition and includes oxides in which constituent elements except oxygen are, by mass%, 10%&lt;Ti, 5%&lt;Al&lt;20% and 5%&lt;Ca&lt;40%. Among oxides, the oxides which have circle-equivalent diameters of less than 2 &mu;m are 300 pieces/mm<SP>2</SP>or more, and the oxides which have circle-equivalent diameters of 2 &mu;m or more are 100 pieces/mm<SP>2</SP>or less. When hardness in a t/4 position is defined as Hvq and hardness in a t/2 position is defined as Hvh, a value H determined by expression of (Hvq-Hvh)/Hvq is 0.07 or less. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a thick steel plate applied to a welded structure such as a bridge, a high-rise building, and a ship. The present invention relates to a thick steel plate having excellent properties.

  In recent years, with the increase in the size of welded structures such as bridges, high-rise buildings, and ships, thick steel plates having a thickness of 50 mm or more are often applied to such welded structures. It is inevitable to weld a thick steel plate with a thickness of. 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 the reduction of 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.

  In addition, an increase in the thickness of the applied thick steel plate increases the difference in cooling rate in the thickness direction in the accelerated cooling process during rolling, resulting in non-uniform strength characteristics in the thickness direction. Will cause the problem. For example, if the cooling rate is increased in order to secure the strength of the central portion of the plate thickness where the cooling rate is the lowest, there will be a problem that the strength on the surface side increases more than necessary. Therefore, even if it is a thick steel plate, the strength on the surface side does not rise more than necessary compared to the central part, and it is possible to develop a thick steel plate with excellent uniformity of strength in the thickness direction, This is another necessary issue.

  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 precipitating fine Ti-containing nitrides as γ-growth growth pinning particles in steel, It is disclosed to suppress the austenite grain coarsening that occurs in HAZ and to suppress degradation of HAZ toughness. However, Ti-containing nitrides tend to disappear when welding heat input is increased, and there is a problem that stable HAZ toughness cannot be obtained, and 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.

  On the other hand, in order to ensure uniformity of strength in the thickness direction of thick steel plates by optimizing rolling, tempering conditions, etc. in the manufacturing process for uneven strength in the thickness direction of thick steel plates Although these techniques are proposed in Patent Document 10 and Patent Document 11, these techniques require extremely complicated processes such as cooling stop and recuperation in the manufacturing process, and are industrially simpler improvements. A measure is desired.

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-A-63-50428 Japanese Patent Laid-Open No. 3-188216

  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, It is another object of the present invention to provide a thick steel plate having excellent uniformity of strength characteristics in the thickness direction.

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: 0.005 to 0.05%, Ti: 0.010 to 0.080%, Ca: 0.0005% to 0.010%, N: 0.002% to 0.020%, the balance being a steel plate with iron and inevitable impurities, the constituent elements excluding oxygen being in mass% 10% <Ti, 5% <Al <20%, 5% <Ca <40% of the 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 existing, HVQ hardness of the plate thickness t / 4 position, the plate thickness t Excellent in toughness and strength uniformity of the heat affected zone, wherein the H value obtained from the formula (Hvq−Hvh) / Hvq is 0.07 or less when the hardness at the 2-position is Hvh. Thick steel plate.

  In addition, including the above description, the equivalent circle diameter described in the present invention refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of the oxide. (TEM) or a scanning electron microscope (SEM).

Invention of Claim 2 is 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: 0.005 to 0.05%, Ti: 0.010 to 0.080%, Ca: 0.0005-0.010%, N: 0.002-0.020% and REM: 0.0001-0.02% and / or Zr: 0.0001-0.02% A thick steel plate containing iron and unavoidable impurities, the constituent elements excluding oxygen being 10% by weight, 10% <Ti, 5% <Al <20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% of an oxide, and the equivalent circle diameter of the oxides 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 existing, HVQ hardness of the plate thickness t / 4 position, the plate thickness t / 2 When the position hardness is Hvh, the H value obtained from the formula (Hvq−Hvh) / Hvq is 0.07 or less, and the toughness and strength uniformity of the weld heat affected zone are excellent. It is a thick steel plate.

In the invention according to claim 3, the constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40%, 5% <REM <50% and / or mass%. 5% <Zr <40%, 10% <REM + Zr <70% is satisfied, and among the oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4, the equivalent circle diameter The thick steel plate excellent in toughness and strength uniformity of the weld heat-affected zone according to claim 2, wherein oxides of less than 2 μm are present in an amount of 300 / mm ² or more.

  The invention according to claim 4 further includes, in mass%, Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0. Thickness excellent in toughness and strength uniformity of weld heat affected zone according to any one of claims 1 to 3, characterized by containing at least one selected from the group consisting of 0.05 to 1.50%. It is a steel plate.

  The invention described in claim 5 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 and strength uniformity of the weld heat-affected zone as described in any of the above.

  The invention according to claim 6 further contains, in mass%, B: 0.0005 to 0.005%, and the toughness of the weld heat affected zone according to any one of claims 1 to 5 and It is a thick steel plate with excellent strength uniformity.

  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, and excellent HAZ toughness is achieved. It is possible to obtain a thick steel plate having excellent uniformity of strength characteristics in the thickness direction.

  First, the inventors examined from various angles for the purpose of ensuring high HAZ toughness with oxide inclusions. Regarding the dispersion of oxide inclusions, the conventional techniques have been constructed on the premise that inclusions with low interfacial energy are effective for intragranular α formation (intragranular α / inclusions). , (Intra-granular α / γ) interface energy is considered to contribute significantly. Accordingly, the present inventors have repeatedly investigated the composition of oxide inclusions that can reduce both the (intragranular α / inclusion) interface energy and the (intragranular α / γ) interface energy. .

  As a result, the constituent elements excluding oxygen contain 10% <Ti, 5% <Al <20%, 5% <Ca <40%, or a predetermined amount of REM or Zr in mass%. In some cases, the oxide satisfying 5% <REM <50% and / or 5% <Zr <40% exhibits a behavior that liquefies during high-temperature heating of HAZ and crystallizes in the subsequent cooling process. In such an oxide, not only (intragranular α / inclusion) interface energy but also (intragranular α / γ) interface energy can be reduced, and the generation of intragranular α is further promoted. I found out.

Of the oxides as described above, the circle equivalent diameter of the oxides of less than 2 [mu] m 300 pieces / mm ² or more, 100 a circle equivalent diameter of more than 2 [mu] m oxide / mm ² or less, so as to present respective By doing so, it was found that both the average value and the minimum value of Charpy absorbed energy showed high levels, and excellent HAZ toughness was obtained.

  In the case where a predetermined amount of REM or Zr is contained, the constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40%, 5% <in mass%. Oxidation in which REM <50% and / or 5% <Zr <40%, 10% <REM + Zr <70% is satisfied, and the mass ratio of Ti and Ca is more than 1 and less than 1.4 In addition to liquefaction during high-temperature heating of HAZ and crystallization with a crystal structure advantageous for intragranular α formation in the subsequent cooling process, in addition to reducing (intragranular α / γ) interface energy, It has also been found that a much lower (intragranular α / inclusion) interfacial energy can be achieved and the generation of intragranular α is promoted very actively.

And it discovered that the more excellent HAZ toughness was obtained by making 300 or more / mm < ² > or more oxides with an equivalent circle diameter less than 2 micrometers among the above oxides exist. The above-mentioned value of REM + Zr indicates the total mass% when REM and Zr are contained in combination, and indicates the single mass% of REM or Zr when REM or Zr is included alone.

  Moreover, it discovered that the intensity | strength characteristic of the thickness direction of a base material can be equalize | homogenized by disperse | distributing these oxide type inclusions in the steel of a base material (thick steel plate). Since oxide inclusions exhibit a certain intragranular α-forming ability even in the base material, the oxide-type inclusions are dispersed in the steel to increase the cooling rate on the surface side of the thick steel plate (for example, the plate thickness). Also at the (t / 4 position), an α structure is formed at a relatively high temperature, and an increase in strength more than necessary can be suppressed.

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

(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. If the equivalent circle diameter of the oxide is 2 μm or more, liquefaction during high-temperature heating of the HAZ does not proceed sufficiently, the production amount of intragranular α decreases, and the HAZ toughness decreases. Further, the composition of the oxide is 10% <Ti, 5% <Al <20%, 5% <Ca <40% (when a predetermined amount of REM or Zr is contained, 5% <REM <50 % And / or 5% <Zr <40%), the liquefaction → crystallization process in HAZ does not proceed, and the intragranular α is not promoted. Further, when the circle equivalent diameter of oxide of less than 2μm is less than 300 / mm ^2, due to the lack of the origin of intragranular α produced, also the amount of intragranular α is reduced, 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 it is preferable that the number is 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.

(H value calculated from (Hvq-Hvh) / Hvq is 0.07 or less)
H value obtained from (Hvq−Hvh) / Hvq, where Hvq is the hardness at the thickness t / 4 position (front side) of the thick steel plate, and Hvh is the hardness at the thickness t / 2 position (center). By setting it to 0.07 or less, it is possible to ensure uniformity of strength characteristics in the thickness direction of the thick steel plate. When this H value exceeds 0.07, the difference between the hardness (Hvq) at the thickness t / 4 position of the thick steel plate and the hardness (Hvh) at the thickness t / 2 position becomes large, and the thickness direction of the thick steel plate increases. It becomes impossible to ensure the uniformity of strength characteristics.

(Production method)
The thick steel plate of the present invention that satisfies the above requirements, in particular, the constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 5% <Ca <40%, 5% <REM in mass%. <50% and / or 5% <Zr <40% of the oxide, and among the oxides, 300 or more oxides / mm ² having an equivalent circle diameter of less than 2 μm and an equivalent circle diameter of In order to manufacture a thick steel plate in which oxides of 2 μm or more exist at 100 pieces / mm ² or less, it is necessary to manufacture the thick steel plate so as to satisfy the following manufacturing requirements.

  The manufacturing requirements are as follows: at the time of melting, the amount of dissolved oxygen in molten steel is 0.002 to 0.01% by mass% by deoxidation using Mn and Si, and then Al → Ti → (REM, Zr →) In order of Ca, while controlling the time t1 from Ti addition to Ca addition to be 3 to 20 minutes, each element was added, and time t2 (min) from Ca addition to the start of casting was This is to keep ta (min) <t2 (min) <tb (min) in a range that satisfies the condition, and to set a cooling time t3 within a temperature range of 1500 to 1450 ° C. during casting within 300 seconds. The reasons for specifying these manufacturing requirements will be described in detail in the following sections.

The ta (minute) and tb (minute) shown above can be obtained from the following calculation formula.
ta = 4-10 × [Ca] / ([Ti] +2 [Al] +5 [REM] +2 [Zr] +0.01)
tb = 25−40 × [Ca] / ([Ti] +2 [Al] +5 [REM] +2 [Zr] +0.01)
However, [Ca], [Ti], [Al], [REM], and [Zr] indicate the amounts (mass%) of Ca, Ti, Al, REM, and Zr added to the molten steel, respectively.

In the case where a predetermined amount of REM or Zr is contained, the constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40%, 5% <REM in mass%. <50% and / or 5% <Zr <40%, 10% <REM + Zr <70% is satisfied, and the mass ratio of Ti and Ca is more than 1 and less than 1.4 Of these, in order to secure 300 oxides / mm ² or more of oxides having an equivalent circle diameter of less than 2 μm, the amount of Ca added [Ca] is determined based on the following formula: A ≦ [Ca] ≦ B It is sufficient to control within the range. The values of A and B obtained based on the following calculation formula are obtained by experiments.

A = 2.25 × [Of]
B = [Of] × [Ti] / (0.25 × [REM] + 0.12 × [Zr])
However, [Of] is the amount of dissolved oxygen (% by mass) before addition of Ca, and [Ti], [REM], and [Zr] are the amounts (% by mass) of addition of Ti, REM, and Zr to the molten steel, respectively. Show.

  That is, when the Ca addition amount [Ca] is less than the A value, most of the added Ca is consumed as an oxide of Ca alone, so that the oxide (the constituent elements are the above requirements) Oxide satisfying (2) cannot be obtained sufficiently. Further, when the Ca addition amount [Ca] exceeds the B value, the Ti / Ca ratio in the oxide becomes less than 1, so that it becomes impossible to secure the necessary number of oxides that are the starting points for intra-granular α formation.

・ Amount of dissolved oxygen in molten steel before addition of Al: 0.002 to 0.01%
When the amount of dissolved oxygen in the molten steel before addition of Al is lower than 0.002%, it becomes impossible to secure a necessary amount of oxide inclusions having an appropriate composition as a starting point of intragranular α formation. On the other hand, when the dissolved oxygen content is higher than 0.01%, coarse inclusions having an equivalent circle diameter of 2 μm or more increase and the HAZ toughness is deteriorated.

-Add in order of Al->Ti-> (REM, Zr->) Ca at the time of melting. Addition of each element in an order other than this order of addition has an oxide-based intervening having an appropriate composition that becomes the starting point of intragranular alpha formation The necessary number of items cannot be secured. In particular, since Ca has a very strong deoxidizing power, if it is added prior to Ti or Al, all of the oxygen associated with Ti and Al will be lost.

-The time t1 from Ti addition to Ca addition is 3 to 20 minutes. If the time t1 from Ti addition to Ca addition is shorter than 3 minutes, the reaction of oxide prior to Ca addition does not proceed sufficiently, and the intra-granular α The required number of oxide inclusions having an appropriate composition as a starting point of generation cannot be secured. Moreover, when this time t1 becomes longer than 20 minutes, the reaction of the oxide prior to Ca addition proceeds excessively, and it becomes impossible to secure the required number of oxide inclusions having an appropriate composition that is the starting point of intragranular α formation. .

・ Time t2 (min) from Ca addition to casting start satisfies ta (min) <t2 (min) <tb (min) Time t2 from Ca addition to casting start depends on oxide generation status This is an influential requirement (time for Ca to take oxygen from other oxides to form oxides), and when this time t2 is less than ta (min), the oxide reaction after Ca addition proceeds sufficiently. Therefore, it becomes impossible to secure the required number of oxide inclusions having an appropriate composition that is the starting point of intragranular α formation. Moreover, when this time t2 becomes tb (min) or more, the reaction of the oxide after the Ca addition proceeds excessively, and the necessary number of oxide inclusions having an appropriate composition that becomes the starting point of intragranular α formation is secured. become unable. The equations for obtaining ta and tb are obtained by experiments in consideration of the ease with which each element becomes an oxide.

・ Cooling time t3 at 1500 to 1450 ° C. during casting is within 300 seconds When the cooling time t3 at 1500 to 1450 ° C. during casting exceeds 300 seconds, formation of coarse oxide inclusions with a circle equivalent diameter of 2 μm or more The amount increases and the HAZ toughness deteriorates.

  Moreover, in order to manufacture a thick steel plate that satisfies the requirement that the H value obtained from (Hvq−Hvh) / Hvq is 0.07 or less, the following requirements are satisfied in the rolling process or the like following these steps. Thus, it is necessary to manufacture a thick steel plate.

  The manufacturing requirements are that the heating temperature prior to rolling is controlled to 1050 to 1200 ° C., the heating time is controlled to 2 to 5 hours, the finishing temperature (FRT) is 900 ° C. or less, and the cooling rate after rolling is the plate thickness. The t / 4 position is 2 to 15 ° C./second, and the cooling stop temperature is 400 to 500 ° C. Further, when the cooling stop temperature is lower than 400 ° C., a tempering process may be performed. The reasons for specifying these manufacturing requirements are as follows.

・ The heating temperature prior to rolling is 1050 to 1200 ° C. and the heating time is 2 to 5 hours. In order to maximize the homogeneity of the α structure in the thickness direction by inclusions, the old γ structure is made uniform before cooling. It is necessary to keep. By setting the heating temperature prior to rolling to 1050 to 1200 ° C. and the heating time to 2 to 5 hours, fine TiN precipitates, the growth of abnormal grains during rolling is suppressed, and a uniform old γ structure is obtained. Can do. When the heating temperature prior to rolling is less than 1050 ° C. or the heating time is less than 2 hours, the precipitation of fine TiN is not sufficiently promoted. On the other hand, when the heating temperature prior to rolling exceeds 1200 ° C. or when the heating time exceeds 5 hours, the Ostwald growth of TiN proceeds and the number of TiN particles decreases.

・ Finishing temperature (FRT) is 900 ° C. or less When the finishing temperature, that is, the rolling end temperature (FRT) is higher than 900 ° C., the accumulated strain before transformation is reduced and the barrier energy for α formation is increased. The generation of intragranular α is not sufficiently promoted.

When the cooling rate after rolling is 2 to 15 ° C./second at the thickness t / 4 position and the cooling rate after rolling is less than 2 ° C./second at the thickness t / 4 position, coarse grain boundary ferrite is generated, The toughness of the base material deteriorates. Further, if the cooling rate after rolling exceeds 15 ° C./second at the plate thickness t / 4 position, the in-grain α cannot be obtained sufficiently, and as a result, the strength characteristics in the plate thickness direction cannot be made uniform.

-Cooling stop temperature is 400-500 ° C
When the cooling stop temperature is lower than 400 ° C., a hard second phase is generated in the intragranular α-untransformed portion near the surface layer of the thick steel plate, and as a result, the strength characteristics in the thickness direction cannot be made uniform. Moreover, when this temperature exceeds 500 degreeC, a soft structure | tissue will increase and intensity | strength will become low entirely.

-Temper treatment Even when the cooling stop temperature is lower than 400 ° C, by applying the temper treatment after the cooling stop, the hard second phase is decomposed, and uneven strength in the thickness direction is improved.

(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 good base metal (thick steel plate) properties and HAZ, if the content of each chemical component (element) is not within the proper range even if the oxide dispersion state is appropriate. I can't. 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, a large amount of hard island martensite (MA) is generated, leading to toughness deterioration of the base material. Therefore, the upper limit of the Si content is 0.25%. Moreover, a preferable upper limit is 0.18% and a more preferable upper limit is 0.05%.

Mn: 1.0-2.0%
Mn is an element useful for securing the strength of the steel sheet, and it is necessary to contain Mn in an amount of 1.0% or more in order to effectively exhibit such effects. 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 ensuring the toughness of the base material and the HAZ, the P content must 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 forms Mn sulfide and degrades the toughness of the base material, its content is preferably as small as possible. From the viewpoint of ensuring the toughness of the base material, 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: 0.005 to 0.05%
Al is an element useful for forming an oxide effective for the generation of intra-granular α by adding prior to Ti, Ca, and REM and Zr added as necessary. In order to exhibit such an effect effectively, it is necessary to contain 0.005% or more. However, if the content is excessive, a coarse oxide is generated and the toughness of the base material and the HAZ deteriorates, so it is necessary to keep it to 0.05% or less. The minimum with preferable content of Al is 0.010%, and a preferable upper limit is 0.04%.

Ti: 0.010 to 0.080%
Ti is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for the formation of intra-granular α by adding prior to Ca and, if necessary, REM and Zr after addition of Al. . In order to exhibit such an effect effectively, it is necessary to make it contain 0.010% or more. However, if the content is excessive, a large amount of coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.080% or less. The preferable lower limit of the Ti content is 0.012%, and the preferable upper limit is 0.060%.

Ca: 0.0005 to 0.010%
Ca is an element that contributes to the improvement of HAZ toughness by forming an effective oxide for the formation of intragranular α by adding Ti and REM and Zr added as necessary after 3 to 20 minutes. It is. 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 toughness of the base material and the HAZ deteriorates, so it is necessary to keep 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 0.020%
N is an element useful for securing the toughness of the base material and the HAZ by forming a nitride (Ti-containing nitride) that remains undissolved at a high temperature. By making the content 0.002% or more, a desired Ti-containing nitride can be secured. However, if its content becomes excessive, the amount of dissolved N increases and the toughness of the base material and HAZ deteriorates due to strain aging, so it is necessary to keep it to 0.020% or less. The preferable lower limit of the N content is 0.003%, and the preferable upper limit is 0.018%.

  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.

REM: 0.0001-0.02% and / or Zr: 0.0001-0.02%
REM (rare earth element) and Zr are elements that contribute to the improvement of HAZ toughness by forming an oxide effective for the generation of intragranular α by adding Ti prior to the addition of Ca. These effects increase as their content increases, but in order to effectively exhibit these effects, it is preferable to contain them in an amount of 0.0001% or more. However, if these are excessively contained, the oxide becomes coarse and deteriorates the toughness of the base material and the HAZ, 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%.

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 toughness of the base material and the HAZ 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 0.005%
B is an element effective for improving the toughness of the base material and the HAZ by suppressing the formation of coarse grain boundaries α. The effect increases as the content increases, but in order to effectively exhibit such an effect, it is preferable to contain 0.0005% or more. However, if the content is excessive, BN precipitation at the austenite grain boundary is caused, and the toughness of the base material and the HAZ is deteriorated. Therefore, it is preferably suppressed to 0.005% or less. A more preferable lower limit of the B content is 0.0010%, a still more preferable lower limit is 0.0015%, and a more preferable upper limit is 0.004%.

  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.

  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] in molten steel before Al addition, Al, Ti, (REM, Zr), Ca addition sequence, time t1 from Ti addition to Ca addition, from Ca addition to casting start. Time t2, cooling time t3 at 1500 to 1450 ° C. during casting, Ca addition amount [Ca], heating temperature Th before rolling, heating time t4 before rolling, rolling end temperature FRT, cooling rate Rc after rolling, cooling stop The temperature Tf is the presence or absence of tempering.

  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 1 and 2, the addition order of Al, Ti, (REM, Zr), and Ca is “◯” when the order of Al → Ti → (REM, Zr) → Ca, and other orders. Time is indicated by “x”. As for the time t2 from the addition of Ca to the start of casting, “o” indicates that the above ta (min) <t2 (min) <tb (min) is satisfied, and “x” indicates that it is not satisfied.

  Regarding the Ca addition amount [Ca], those satisfying the relationship of A ≦ [Ca] ≦ B described above are indicated by “◯”, and those not satisfied are indicated by “X”, and do not contain REM and Zr. Items not related to are indicated by “−”. In addition, when the cooling stop temperature is lower than 400 ° C., “○” indicates that the temper treatment was performed after the cooling stop, “×” indicates that the temper treatment was not performed, and the cooling stop temperature is 400 ° C. or higher and does not relate to this requirement Is indicated by “−”.

  Using each hot-rolled sheet (thick steel plate) manufactured according to the above requirements, the number density of various sized oxides (oxide inclusions), HAZ toughness, and uniformity of strength in the sheet thickness direction (H value) , T value) was obtained by measurement. 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 μm 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. 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.

Further, among the measured oxides, the constituent elements containing a predetermined amount of REM and Zr and excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40 in mass%. %, 5% <REM <50% and / or 5% <Zr <40% and 10% <REM + Zr <70%, and the mass ratio of Ti and Ca is more than 1. The number (N3) of oxides having an equivalent circle diameter of less than 2 μm with an oxide of less than 4 was converted to a number density equivalent to 1 mm ² . (Note that this N3 value, a predetermined amount of REM or Zr, and the constituent elements excluding oxygen are mass%, the constituent elements excluding oxygen are mass%, and 10% <Ti, 5% <Al < (N1 is the sum of 20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40%.)

(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 μm 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. 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 .

(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 piece (V notch of JIS Z 2202) in which a notch was machined in the HAZ near the weld line (bond) at a position of 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.

Moreover, the Charpy impact test was performed under the same conditions as described above except that electrogas arc welding was performed at an amount of 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.

(Evaluation of strength uniformity in the thickness direction by H value)
The hardness Hvq at the thickness t / 4 position (surface side) is obtained by measuring the hardness at the thickness t / 4 position and the t / 4 ± 2 mm position by measuring the Vickers hardness with a load of 10 kg, and calculating the average value of the three points. Asked. Similarly, the hardness Hvh at the plate thickness t / 2 position (central portion) is obtained by measuring the plate thickness t / 2 position and the hardness at the t / 2 ± 2 mm position by measuring the Vickers hardness with a load of 10 kg, respectively. The average value was obtained. Using the average value obtained by these measurements, the H value was obtained from the formula (Hvq−Hvh) / Hvq. Those having an H value of 0.07 or less were evaluated as having excellent strength uniformity in the thickness direction.

(Evaluation of strength uniformity in the thickness direction by T value)
JIS Z 2201 No. 4 specimens were sampled at right angles to the rolling direction from the thickness t / 4 position and thickness t / 2 position of each thick steel plate (as-rolled material), and a tensile test of JIS Z 2241 was carried out. Then, the tensile strength TS at the t / 4 position and the t / 2 position was obtained, respectively. The tensile strength TS at the t / 4 position determined by the tensile test was TSq, the tensile strength TS at the t / 2 position was TSh, and the T value was determined from the formula (TSq−TSh) / TSq. Those having a T value of 0.095 or less were evaluated as having excellent strength uniformity in the thickness direction.

  No. 1-28 are examples of the invention that satisfy the requirements of the present invention, where the chemical composition, oxide dispersion, etc. are appropriately made, and the HAZ toughness (average value and minimum value) when the heat input is 50 kJ / mm Value), and the uniformity of strength in the plate thickness direction confirmed by the H value and the T value is found to be excellent. That is, no. 1-28 can be said to be thick steel plates excellent in toughness and strength uniformity of the weld heat affected zone.

  In particular, the constituent elements containing a predetermined amount of REM or Zr and excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40%, 5% <REM <50 in mass%. % And / or 5% <Zr <40%, 10% <REM + Zr <70% is satisfied, and the mass ratio of Ti and Ca is more than 1 and less than 1.4, HAZ toughness when the number of oxides with an equivalent circle diameter of less than 2 μm (N3) is 300 or more (No. 6-8, 13, 14, 21-24) when the heat input is 60 kJ / mm It can be seen that it is excellent.

  In contrast, no. Nos. 29 to 50 are comparative examples that do not satisfy any of the requirements of the present invention, and either the average value or the minimum value of the HAZ toughness or the strength in the thickness direction confirmed by the H value or the T value. It can be seen that the uniformity does not satisfy the evaluation criteria.

Claims (6)

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: 0.005-0.05%, Ti: 0.010-0.080%, Ca: 0.0005-0. 0.10%, N: 0.002 to 0.020%, the balance is a steel plate with iron and inevitable impurities,
Constituent elements excluding oxygen contain, in mass%, an oxide of 10% <Ti, 5% <Al <20%, 5% <Ca <40%, and among the oxides, the equivalent circle diameter There are 300 oxides / mm ² or more of oxides having a diameter of less than 2 μm and 100 / mm ² or less of oxides having an equivalent circle diameter of 2 μm or more,
When the hardness at the plate thickness t / 4 position is Hvq and the hardness at the plate thickness t / 2 position is Hvh, the H value obtained from the formula (Hvq−Hvh) / Hvq is 0.07 or less. A thick steel plate with excellent toughness and strength uniformity in the weld heat affected zone. 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: 0.005-0.05%, Ti: 0.010-0.080%, Ca: 0.0005-0. 010%, N: 0.002 to 0.020% and REM: 0.0001 to 0.02% and / or Zr: 0.0001 to 0.02%, the balance being iron and inevitable Steel plate, which is a typical impurity,
Constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% in mass%. containing certain oxides, and said one of the oxide, circles oxide equivalent diameter of less than 2μm is 300 / mm ² or more, the oxide equivalent circle diameter of more than 2μm is 100 / mm ² or less, there As well as
When the hardness at the plate thickness t / 4 position is Hvq and the hardness at the plate thickness t / 2 position is Hvh, the H value obtained from the formula (Hvq−Hvh) / Hvq is 0.07 or less. A thick steel plate with excellent toughness and strength uniformity in the weld heat affected zone. Constituent elements excluding oxygen are 10% <Ti, 5% <Al <20%, 8% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% in mass%. Further, among the oxides satisfying 10% <REM + Zr <70% and having a mass ratio of Ti and Ca of more than 1 and less than 1.4, oxides having an equivalent circle diameter of less than 2 μm are 300. The thick steel plate excellent in toughness and strength uniformity of the weld heat-affected zone according to claim ^{2, wherein} there are at least pieces / mm ² .   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 having excellent toughness and strength uniformity of the weld heat affected zone according to any one of claims 1 to 3, wherein the steel plate contains at least one selected from the group consisting of:   The welding heat according to any one of claims 1 to 4, further comprising Nb: 0.002 to 0.10% and / or V: 0.002 to 0.10% in mass%. A thick steel plate with excellent toughness and strength uniformity in the affected area. Furthermore, B: 0.0005 to 0.005% in mass%, The thickness excellent in toughness and strength uniformity of the weld heat affected zone according to any one of claims 1 to 5 steel sheet.