JP2012092425A - Thick steel plate excellent in toughness of weld heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate excellent in toughness of a weld heat-affected zone and capable of improving HAZ toughness when large energy welding is performed in a high-strength steel plate having a tensile strength of 80 kg-class.SOLUTION: The thick steel plate: satisfies a predefined chemical composition; has a value of D obtained from formula: D=62×[Mn]+27×[Ni]+111×[Cr], of more than 238 and less than 388; and includes oxides whose constituting elements other than oxygen consist of Ti in a content more than 10 mass%, Al in a content less than 20 mass%, Ca in a content more than 5 mass% and less than 40 mass%, REM in a content more than 5 mass% and less than 50 mass%, and Zr in a content more than 5 mass% and less than 40 mass%. Among the oxides, the number of oxides each having an equivalent circle diameter less than 2 μm is 300 pieces/mmor more; the number of oxides each having an equivalent circle diameter of 2 μm or more and less than 5 μm is 30-70 pieces/mm; and the number of oxides each having an equivalent circle diameter of 5 μm or more is less than 30 pieces/mm.

Description

  The present invention relates to a thick steel plate mainly applied to a high-rise building, and more particularly to a thick steel plate excellent in the strength and toughness of a heat-affected zone after large heat input (hereinafter also referred to as HAZ). .

  In recent years, with the increase in the height and size of buildings, it has been required to increase the strength and thickness of the steel used, and in a high-strength thick steel plate with a tensile strength exceeding 80 kg, a thickness of 50 mm or more The welding of thick steel plates 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 HAZ toughness during high heat input welding. In addition, toughness generally lowers toughness, so we developed a technology to stably maintain the strength of thick steel plates and the toughness of HAZ during high heat input welding (hereinafter also referred to as HAZ toughness). It has become a necessary task.

  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.

  That is, the amount of heat input that is assumed in both the techniques described in Patent Document 1 and Patent Document 2 remains at a low level. Moreover, although the specific tensile strength of welded structural steel is not specified in Patent Document 1, when estimated from the concentration of the added alloying element, the tensile strength is at a relatively low level that does not reach 80 kg class. It seems that it has remained.

  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 the generation of intragranular bainite (hereinafter also referred to as IGB) starting from inclusion particles, the inventors have disclosed inclusion compositions and inclusion shapes in Patent Document 7 and Patent Document 8. The technology which accelerates | stimulates IGB production | generation is proposed by controlling this. However, the strength classes of the thick steel plates proposed in these Patent Documents 7 and 8 remain at a relatively low level of 50 to 60 kg, and both high strength and HAZ toughness during high heat input welding are compatible. It's hard to say.

Furthermore, in high-strength thick steel plates, a large amount of alloying elements are added to ensure strength, but increasing the concentration of alloying elements tends to lower the temperature at which the driving force for IGB generation begins to work (T ₀ temperature). This is disadvantageous for the generation of IGB. Therefore, in the high-strength thick steel plate, it can be said that in order to obtain IGB generation starting from inclusions, it is necessary to appropriately control the component composition of the alloy as well as to form a dispersion form of inclusions that are further excellent in IGB generation ability. .

  Further, as a technique for obtaining IGB generation starting from inclusions in a high-strength thick steel sheet having a tensile strength of 60 to 80 kilo class, Patent Document 9 proposes a technique using Ti oxide. However, in the technique described in Patent Document 9, as a manufacturing method for obtaining a prescribed Ti oxide dispersion form, it has been proposed to hold in a stationary state for a long time after addition of Ti at the melting stage. It is assumed that the manufacturing cost is significantly increased. Furthermore, in the technique described in Patent Document 9, the HAZ toughness is evaluated by obtaining an average value of Charpy impact energy. However, from the viewpoint of safety, it is not sufficient to obtain only the average value of Charpy impact energy, and it is considered necessary to evaluate with the minimum value of Charpy impact energy.

JP 2001-98340 A JP 2008-308336 A Japanese Patent Laid-Open No. 2001-20031 JP 2007-247005 A JP 2008-223062 A JP 2009-179844 A JP 2010-168644 A JP 2008-223081 A JP 2006-124759 A

  The present invention has been made in view of the above-described conventional circumstances, and improves the minimum value and further the average value of HAZ toughness when performing high heat input welding in a high-strength steel sheet having a tensile strength of 80 kg class. It is an object of the present invention to provide a thick steel plate excellent in toughness of a weld heat affected zone.

Invention of Claim 1 is the mass%, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 1.4-3.0%, P: 0.03 % Or less (excluding 0%), S: 0.015% or less (not including 0%), Al: 0.07% or less (including 0%), Cr: 0.5 to 2.0%, Ti: 0.010-0.080%, REM: 0.0003-0.02%, Zr: 0.0003-0.02%, Ca: 0.0005-0.010%, N: 0.002- It is a thick steel plate containing 0.020%, the balance being iron and inevitable impurities, and the D value obtained from the equation D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr] is 238 <D <388 and the constituent elements excluding oxygen are 10% <Ti, Al <20%, 5% <Ca <40%, 5% by mass. <REM <50% contain 5% <Zr <oxides is 40%, and, among the oxides, oxides of less than the circle equivalent diameter of 2μm is 300 / mm ² or more, the equivalent circle diameter oxides of less than 5μm or more 2μm 30-70 pieces / mm ^2, the steel plate of the circle equivalent diameter is superior in toughness of the heat affected zone more oxide 5μm is characterized by the presence of less than 30 / mm ² It is.
However, in the above formula, [] indicates the content (% by mass) of each element.

  In addition, including the above description, the equivalent circle diameter described in the present invention refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of an oxide or the like. It can obtain | require by observing with a 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%, 10% <Ti, Al <20%, 8% <Ca <40%, 5% <REM <50%, 5% <Zr <. 40% and satisfying 10% <REM + Zr <70%, and among oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4, the equivalent circle diameter is less than 2 μm. The thick steel plate having excellent toughness of the weld heat affected zone according to claim 1, wherein the number of objects is 300 / mm ² or more.

The invention according to claim 3 further includes 5.0 × 10 ⁶ pieces / mm ² or more of Ti-containing nitride having an equivalent circle diameter of less than 0.05 μm. It is a thick steel plate with excellent toughness of the weld heat affected zone.

  The invention according to claim 4 further includes, in mass%, Ni: 0.05-2.0%, Cu: 0.05-2.0%, Mo: 0.05-2.0%. It is a thick steel plate excellent in the toughness of the welding heat affected zone in any one of Claims 1 thru | or 3 characterized by containing 1 or more types chosen.

  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 of the welding heat affected zone in any one of.

  The invention according to claim 6 further includes B: 0.0003 to 0.005% by mass%, and the toughness of the weld heat affected zone according to any one of claims 1 to 5 It is an excellent thick steel plate.

  According to the present invention, in a high strength steel plate with a tensile strength of 80 kg class, the minimum value or average value of the HAZ toughness can be obtained not only for small to medium heat input welding but also for large heat input welding. It can be improved and excellent HAZ toughness can be obtained.

  First, the inventors studied for the purpose of finding an oxide form excellent in the ability to produce intragranular bainite (IGB). The inventors have already found an oxide composition advantageous for IGB generation from the viewpoint of interfacial energy in Patent Document 7, and in addition to that, investigated the influence of oxide size on IGB generation.

  Conventionally, it has been thought that the coarsening of the oxide uniformly has an adverse effect on the HAZ toughness. However, as a result of this investigation, it was found that the coarsening of the oxide has the effect of promoting IGB generation, and that the allowable oxide size increases as the temperature at which HAZ toughness is evaluated rises. In addition, for HAZ toughness at 0 ° C., which is mainly required for heavy steel plates for construction, among oxides having an equivalent circle diameter of 2 μm or more, an oxide having an equivalent circle diameter of 2 μm or more and less than 5 μm is predetermined. It has been clarified that excellent HAZ toughness can be obtained by performing number dispersion and suppressing the number of oxides of 5 μm or more to less than a certain number.

In general, the addition of an alloy element for ensuring strength tends to lower the temperature (T ₀ temperature) at which the driving force for IGB generation starts to work, and is considered to be disadvantageous for IGB generation. On the other hand, the inventors pay attention to the difference between the T ₀ temperature and the temperature at which IGB generation actually starts ( _TIGB temperature), and by adding the alloying element in a controlled manner based on an appropriate balance, even if T It has been found that sufficient IGB generation can be obtained even at a low ₀ temperature. That is, by controlling the alloy element based on the D value, the decrease in the T ₀ temperature is minimized, and the generation of grain boundary bainite competing with the IGB is suppressed, and the difference between the T ₀ temperature and the T _IGB temperature is reduced. By enlarging, the driving force of the IGB generation capability increases and sufficient IGB generation is obtained.

  As described above, in addition to controlling the oxide composition and size, by carrying out balance control of the alloy elements, the HAZ toughness when performing high heat input welding on a high strength steel sheet with a tensile strength of 80 kg class. It becomes possible to improve the minimum value. The present invention has been completed on the basis of the knowledge described above. The reasons for defining each constituent element are as follows. In addition, the high strength steel plate having a tensile strength of 80 kg described in the present specification specifically indicates a high strength steel plate having a tensile strength exceeding 780 MPa.

(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 the generation of IGB. 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 amount of IGB generated decreases, and the HAZ toughness decreases. If the composition of the oxide is out of the range of 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50%, 5% <Zr <40%, the liquid state in HAZ → The crystallization process does not proceed and IGB generation is not promoted. On the other hand, if the number of oxides having an equivalent circle diameter of less than 2 μm is less than 300 / mm ² , the starting point of IGB generation is insufficient, so that the amount of IGB generation is also reduced and sufficient HAZ toughness cannot be obtained.

(30-70 oxides / mm ^{2 with an} equivalent circle diameter of 2 μm or more and less than 5 μm)
Oxides of less than the circle equivalent diameter of 2 [mu] m or more 5μm, since the equivalent circle diameter having the composition specified which is the is high ability of producing IGB compared to oxide of less than 2 [mu] m, by dispersing 30 / mm ² or more Even in a high-strength steel sheet with a tensile strength of 80 kg class, there is a remarkable effect in refining the HAZ structure. On the other hand, when the number density exceeds 70 pieces / mm ² , an adverse effect as a brittle fracture starting point becomes obvious, so it is necessary to control the number density to 70 pieces / mm ² or less.

(Equivalent circle diameter is less than 30 oxide / mm ² with 5 μm or more)
An oxide having an equivalent circle diameter of 5 μm or more has a great adverse effect on the HAZ toughness as a brittle fracture starting point, and therefore needs to be controlled to be less than 30 pieces / mm ² .

(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, Al <20%, 5% <Ca <40%, 5% <REM <50% in mass%. 5% <Zr <40% is included, and among the oxides, 300 / mm ² or more of oxides with an equivalent circle diameter of less than 2 μm and an equivalent circle diameter of 2 to 5 μm In order to produce a thick steel plate having 30 to 70 oxides / mm ² and an equivalent circle diameter of 5 μm or more and less than 30 oxides / mm ² , respectively, the following production requirements must be satisfied: It is necessary to manufacture a thick steel plate.

  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, In order of Zr) → Ca, the time t1 from the addition of Ti to the addition of Ca is controlled so as to be 3 to 20 minutes, each element is added, and the time t2 (minutes) from the addition of Ca to the start of casting is 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. At this time, the addition amount of Ti, REM, and Zr must be a value obtained by [Ti] / ([REM] + [Zr]) of 0.8 or more and less than 11.8. In the above formula, [] indicates the amount (mass%) of each element added to the molten steel. 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.

Further, the constituent elements excluding oxygen are 10% <Ti, Al <20%, 8% <Ca <40%, 5% <REM <50%, 5% <Zr <40% in mass%, And 10% <REM + Zr <70% is satisfied, and furthermore, among oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4, 300 oxides / mm having an equivalent circle diameter of less than 2 μm / mm ^In order to secure the condition of ² or more, the additive amount [Ca] of Ca may be controlled within a range of A ≦ [Ca] ≦ B obtained based on the following calculation formula. 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 that serves as the starting point of IGB generation (the constituent elements satisfy the above requirements) Oxide) 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 the generation of IGB.

・ 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 the addition of Al is lower than 0.002%, it becomes impossible to secure a necessary amount of oxide inclusions having an appropriate composition that is the starting point of IGB generation. 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.

At the time of melting, the addition of Al → Ti → (REM, Zr) → Ca in this order When each element is added in an order other than this addition order, an oxide inclusion having an appropriate composition that becomes the starting point of IGB generation The necessary number 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.

・ Time t1 from Ti addition to Ca addition is 3 to 20 minutes If 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 IGB generation The required number of oxide inclusions having an appropriate composition as a starting point 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 becomes the starting point of IGB generation.

・ 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, the required number of oxide inclusions having an appropriate composition that is the starting point of IGB generation cannot be secured. Moreover, when this time t2 becomes tb (min) or more, the reaction of the oxide after the Ca addition proceeds excessively, and it becomes impossible to secure the required number of oxide inclusions having an appropriate composition that becomes the starting point of IGB generation. . 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 If 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 5 μm or more The amount increases and the HAZ toughness deteriorates.

Addition amount of Ti, REM, Zr: The value obtained by [Ti] / ([REM] + [Zr]) is 0.8 or more and less than 11.8 [Ti] / ([REM] + [Zr]) When the required value is less than 0.8, the amount of addition of REM and Zr, which are strong deoxidation elements, is larger than that of Ti, which is a weak deoxidation element. With such an addition amount, the free oxygen concentration in the molten steel decreases, and the oxide growth rate during subsequent Ca addition decreases. Therefore, 30 oxides / mm ² with an equivalent circle diameter of 2 μm or more and less than 5 μm. No more can be secured. On the other hand, when the value obtained by [Ti] / ([REM] + [Zr]) is 11.8 or more, the number of oxides having an equivalent circle diameter of 2 μm or more and less than 5 μm exceeds 70 / mm ^2. .

  In producing the thick steel plate of the present invention, the production method other than the requirements in the smelting to casting steps described above is not particularly limited, but after heating the obtained slab, hot rolling is performed, After quenching, it is recommended that the austenite / ferrite two-phase region be reheated and quenched and tempered.

(Ti-containing nitride is 5.0 × 10 ⁶ pieces / mm ² or more)
Further, in addition to controlling the oxide, the inventors contain a suitable number of fine Ti-containing nitrides in the thick steel plate, thereby suppressing austenite grain coarsening during the HAZ thermal cycle by pinning. It has been found that by making the HAZ structure finer, it can contribute to the improvement of toughness, in particular, the average value of the HAZ toughness can be improved.

  The Ti-containing nitride defined in the present invention includes not only TiN but also a part of Ti of TiN, specifically, an element having an atomic ratio of 50% or less in place of Ti and other nitride-forming elements. Those substituted with (Nb, Zr, V, etc.) are also included.

  This Ti-containing nitride is very fine as compared with the oxide, and since it can be finely dispersed in the steel before heat input by welding, even if the heat input is 100 kJ / mm or more Even if much of the Ti-containing nitride disappears due to such high heat input welding, a sufficient number of Ti-containing nitrides can still remain. In this way, a sufficient number of Ti-containing nitrides can still remain after high heat input welding, so that the pinning action of HAZ austenite grain coarsening can be effectively exhibited.

In order to ensure HAZ toughness more reliably, it is preferable to contain 5.0 × 10 ⁶ pieces / mm ² or more of Ti-containing nitride having an equivalent circle diameter of less than 0.05 μm. When the equivalent circle diameter of the Ti-containing nitride is 0.05 μm or more, the pinning action of HAZ austenite grain coarsening is reduced. If the number density of Ti-containing nitrides having an equivalent circle diameter of less than 0.05 μm is less than 5.0 × 10 ⁶ pieces / mm ² , the pinning action of austenite grain coarsening cannot be effectively exhibited. More preferably, it is 8.0 × 10 ⁶ or more.

  In the present invention, the lower limit of the equivalent circle diameter of the fine Ti-containing nitride introduced into the steel material is not particularly specified, but from the measurement limit of a transmission electron microscope (TEM) for measurement, the actual equivalent circle diameter is determined. Can be said to be about 0.01 μm.

(Production method)
In order to produce a thick steel plate of the present invention that satisfies the above-described requirements, that is, a steel plate containing 5.0 × 10 ⁶ pieces / mm ² or more of Ti-containing nitride having an equivalent circle diameter of less than 0.05 μm, In addition to the manufacturing requirements described above, it is necessary to manufacture a thick steel plate so as to satisfy the following manufacturing requirements.

  The manufacturing requirements are that the heating temperature before rolling is 1050 to 1200 ° C., and the rolling reduction of 900 ° C. or more is 40% or more. The reasons for defining these manufacturing requirements will be explained in detail in the following sections.

・ The heating temperature before rolling is 1050 to 1200 ° C.
When the heating temperature before rolling is lower than 1050 ° C., a sufficient number of Ti-containing nitrides are not precipitated. On the other hand, when the heating temperature before rolling exceeds 1200 ° C., the Ti-containing nitride is coarsened and particles having a predetermined size cannot be sufficiently obtained.

-Rolling rate of 900 ° C or higher is 40% or more Ti-containing nitride is also precipitated during rolling during strain induction. When the rolling reduction at 900 ° C. or higher is less than 40%, the amount of strain becomes insufficient and a predetermined dispersion cannot be obtained. Further, if the temperature is lower than 900 ° C., Ti diffusion does not proceed sufficiently, so that even if the rolling reduction is ensured, a predetermined dispersion cannot be obtained in the same manner.

(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 is suitable for the base material (thick steel plate) if the content of each chemical component (element) is not within the proper range even if the oxide, Ti-containing nitride dispersion state is appropriate. Characteristics 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: 0.02 to 0.50%
Si is an effective element for ensuring the strength of the steel sheet. Moreover, there exists an effect | action which raises Ti activity, and addition of a small quantity promotes the fine dispersion | distribution of TiN effective for the improvement of HAZ toughness. However, if it is added in excess, a large amount of hard island martensite (MA) is produced, which adversely affects the HAZ toughness. Therefore, the Si content is 0.02 to 0.50%. A preferred lower limit is 0.05%, a more preferred lower limit is 0.10%, a still more preferred lower limit is 0.20%, a preferred upper limit is 0.40%, and a more preferred upper limit is 0.30%.

Mn: 1.4 to 3.0%
Mn is an element useful for ensuring the strength of the steel sheet, and it is necessary to contain 1.4% or more in order to exert such an effect effectively. However, if the content exceeds 3.0%, the HAZ strength increases excessively and adversely affects the HAZ toughness. Therefore, the Mn content is set to 1.4 to 3.0%. A preferred lower limit is 1.5%, a more preferred lower limit is 1.6%, a still more preferred lower limit is 1.8%, a preferred upper limit is 2.8%, and a more preferred upper limit is 2.5%.

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: 0.07% or less (including 0%)
Al is an element useful for forming an oxide effective for generating IGB by adding it prior to Ti, Ca, REM, and Zr. 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 0.07% or less. The preferable lower limit of the Al content is 0.005%, more preferably 0.01%, the preferable upper limit is 0.06%, and the more preferable upper limit is 0.04%.

Cr: 0.5 to 2.0%
Cr is an effective element for securing the strength of the steel sheet, and it is necessary to add 0.5% or more for securing the strength. However, if added excessively, the HAZ strength will be excessively increased and the toughness of the HAZ will be adversely affected, so it is necessary to keep it at 2.0% or less. The preferable lower limit of the Cr content is 0.6%, more preferably 0.7%, the preferable upper limit is 1.8%, and the more preferable upper limit is 1.6%.

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 generation of IGB by adding prior to Ca, REM and Zr after the 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%.

REM (rare earth element): 0.0003 to 0.02%
REM (rare earth element) is an element that forms an oxide effective for the generation of IGB and contributes to the improvement of HAZ toughness by being added prior to the addition of Ca after the addition of Ti. Such effects increase as their content increases, but in order to effectively exhibit these effects, it is necessary to contain 0.0003% or more. However, if it is excessively contained, the oxide becomes coarse and deteriorates the toughness of the base material and the HAZ, so it should be suppressed to 0.02% or less. The preferable lower limit of the REM content is 0.0005%, and the preferable upper limit is 0.015%.

Zr: 0.0003 to 0.02%
Zr is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for the generation of IGB by adding Ti prior to the addition of Ca, similar to REM. Such effects increase as their content increases, but in order to effectively exhibit these effects, it is necessary to contain 0.0003% or more. However, if it is excessively contained, the oxide becomes coarse and deteriorates the toughness of the base material and the HAZ, so it should be suppressed to 0.02% or less. The preferable lower limit of the Zr content is 0.0005%, and the preferable upper limit is 0.015%.

Ca: 0.0005 to 0.010%
Ca is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for the generation of IGB by adding 3 to 20 minutes after the addition of Ti, REM, and Zr. 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.

One or more selected from the group consisting of Ni: 0.05-2.0%, Cu :: 0.05-2.0%, Mo: 0.05-2.0% Ni, Cu, and Mo are: All are effective elements for increasing the strength of steel sheets, and the effect increases as their content 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 adversely affected. The more preferable lower limit of the content of these elements is 0.1% for all, the more preferable lower limit is 0.15% for all, the more preferable upper limit is 1.8%, and the more preferable upper limit is any. Is 1.6%.

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 is 0.005% for all, and the preferable upper limit is 0.08% for all.

B: 0.0003 to 0.005%
B is an element effective in improving the toughness of the base material and the HAZ by suppressing the formation of coarse IGB. The effect increases as the content increases, but in order to effectively exhibit such an effect, it is preferable to contain 0.0003% 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 0.003% or less. The more preferable lower limit of the B content is 0.0005%, the still more preferable lower limit is 0.0010%, the most preferable lower limit is 0.0015%, and the more preferable upper limit is 0.004%.

(D value)
After satisfying the above chemical composition, the thick steel plate of the present invention has a D value calculated from the formula of D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr] such that 238 <D <388. It is necessary to satisfy (however, in the above formula, [] indicates the content (% by mass) of each element). When this D value is lower than 238, the oxide-origin IGB generation cannot be sufficiently obtained, and the HAZ toughness is lowered. On the other hand, if the D value is larger than 388, the strength of the HAZ increases excessively and the HAZ toughness cannot be secured.

  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.

Example 1
In Example 1, first, steels having respective component compositions shown in Tables 1 and 2 (in addition, Al, Ti, REM, Zr, and Ca indicate the mass% after addition) are used in a vacuum melting furnace. (VIF: 150 kg) After melting, cast slab (cross-sectional shape: 150 mm × 250 mm) using the molten steel, and further hot rolling using the slab, heat of 50 mm thickness A rolled sheet was obtained.

  Tables 3 and 4 show the controlled conditions in producing this hot-rolled sheet (thick steel sheet). The conditions include the amount of dissolved oxygen [O] in the molten steel before Al addition, the order of addition of Al, Ti, REM, Zr, and Ca, [Ti] / ([REM] + [Zr]), and addition of Ti to addition of Ca Time t1, time t2 from the addition of Ca to the start of casting, cooling time t3 at 1500 to 1450 ° C. during casting, and Ca addition amount [Ca].

  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 order of addition of Al, Ti, REM, Zr, and Ca is “◯” when the order of Al → Ti → (REM, Zr) → Ca, and other orders. 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.

  As for the Ca addition amount [Ca], “◯” indicates that the relationship of A ≦ [Ca] ≦ B described above is satisfied, and “×” indicates that the relationship is not satisfied.

  Using each hot-rolled sheet (thick steel plate) manufactured according to the above requirements, the number density, HAZ toughness (minimum value), and tensile strength of various sized oxides (oxide inclusions) are obtained by measurement. did. These measurement results are shown in Tables 3 and 4.

(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 excluding oxygen are 10% <Ti, Al <20%, 8% <Ca <40%, 5% <REM <50%, 5% <% by mass. Zr <40%, 10% <REM + Zr <70% is satisfied, and among oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4, the equivalent circle diameter is less than 2 μm The number (NA) of oxides was determined by converting the number density to 1 mm ² .

(Measurement of number density of oxides with equivalent circle diameter of 2 μm or more and less than 5 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. 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 of oxides (N2) having an equivalent circle diameter of 2 μm or more and less than 5 μm was calculated by converting it to a number density corresponding to 1 mm ² .

(Measurement of number density of oxides with equivalent circle diameter of 5 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 (N3) of oxides having an equivalent circle diameter of 5 μm or more was determined by converting into a number density equivalent to 1 mm ² .

(Evaluation of HAZ toughness)
A test piece of 12.5 × 32 × 55 mm was cut out from the position of depth t / 4 (t: plate thickness) from the surface of each thick steel plate, held at 1400 ° C. for 5 seconds, and then cooled to 800 ° C. to 500 ° C. Cooling was performed by controlling the speed so that the time was 200 seconds. This is a thermal cycle simulating corner joint submerged arc welding (heat input: 50 kJ / mm). From these test pieces, three Charpy impact test pieces (JIS Z 2202 V-notch test pieces) were sampled and subjected to a Charpy impact test at 0 ° C. to measure the absorbed energy vE ₀ . From each of these three Charpy impact test measurement results, those having a minimum absorbed energy vE ₀ (min) exceeding 70 J were evaluated as having excellent HAZ toughness.

(Evaluation of tensile strength)
From the position of depth t / 4 (t: plate thickness) from the surface of each thick steel plate, JIS Z 2201 No. 4 test piece was sampled at right angles to the rolling direction, and a tensile test of JIS Z 2241 was carried out for The strength TS was determined. Those satisfying TS> 780 MPa were evaluated as having excellent strength.

  No. 1-32 are invention examples that satisfy the requirements of the present invention, and it is understood that chemical component composition, oxide dispersion, etc. are appropriately performed, and that HAZ toughness and strength are excellent. That is, no. 1-32 can be said to be thick steel plates excellent in the strength and toughness of HAZ after large heat input.

In general, HAZ toughness and tensile strength tend to show a negative correlation. Among the inventive examples that satisfy the requirements of the present invention, No. 1 having a tensile strength of 810 to 820 MPa. 2, 6, and 32, the oxide number NA does not satisfy the requirement of claim 2. Only 6 has E ₀ (min) below 110 kJ.

  In contrast, no. 33 to 64 are comparative examples that do not satisfy any of the requirements of the present invention, and it can be seen that at least one of HAZ toughness and strength does not satisfy the evaluation criteria.

(Example 2)
In Example 2, No. 1 in Example 1 was obtained. 1, 6, 26, and steels having substantially the same component composition (shown in Table 5. In addition, Al, Ti, REM, Zr, and Ca are described in terms of mass% after addition). After melting with a melting furnace (VIF: 150 kg), a cast slab (cross-sectional shape: 150 mm × 250 mm) is cast using the molten steel, and hot rolling is performed as shown in Table 6 using the slab. Thus, a hot-rolled sheet having a thickness of 50 mm was obtained.

  Table 6 shows the controlled conditions in producing this hot-rolled sheet (thick steel sheet). The conditions include the amount of dissolved oxygen [O] in the molten steel before Al addition, the order of addition of Al, Ti, REM, Zr, and Ca, [Ti] / ([REM] + [Zr]), and addition of Ti to addition of Ca In addition to the time t1, the time t2 from the Ca addition to the start of casting, the cooling time t3 at 1500 to 1450 ° C. during casting, the Ca addition amount [Ca], the heating temperature before rolling, the rolling reduction at 900 ° C. or higher It is.

  In Table 5, REM was added in the form of a misch metal containing about 50% Ce and about 25% La in the same manner as in Example 1. In Table 5, “-” indicates that the corresponding element is not added.

  In Table 6, the order of addition of Al, Ti, REM, Zr, and Ca is “◯” when Al → Ti → (REM, Zr) → Ca, and “X” when other orders are added. It shows with. 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.

  As for the Ca addition amount [Ca], “◯” indicates that the relationship of A ≦ [Ca] ≦ B described above is satisfied, and “×” indicates that the relationship is not satisfied.

  Using each hot-rolled sheet (thick steel plate) manufactured according to the above requirements, the number density of oxides (oxide inclusions) of various sizes, the Ti-containing nitride having an equivalent circle diameter of less than 0.05 μm The number density, HAZ toughness (minimum value and average value), and tensile strength were determined by measurement. These measurement results are shown in Table 7.

  In addition, the evaluation of the minimum value among the measurement of the number density of oxides (oxide inclusions) of various sizes, the evaluation of the tensile strength, and the evaluation of the HAZ toughness is the same as in Example 1 described above.

(Measurement of the number density of Ti-containing nitrides having an equivalent circle diameter of less than 0.05 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. A TEM replica test piece was prepared from the parallel cross section, and the cross section was observed using a transmission electron microscope (TEM). The observation conditions were magnification: 150,000 times, observation field: 0.66 μm × 0.78 μm, and four fields of view were observed. And the particle | grains containing Ti and N were discriminate | determined by EDX (energy dispersive X-ray detector), and the particle | grains were made into Ti containing nitride. Further, by image analysis, the area of the Ti-containing nitride in this observation field is measured, the number of Ti-containing nitrides less than 0.05 μm is measured in terms of the equivalent circle diameter, and the number density is equivalent to 1 mm ^2. And asked. However, particles having an equivalent circle diameter of 0.01 μm or less were excluded from the analysis because the reliability of EDX was not sufficient.

(Evaluation of HAZ toughness)
Among the evaluations of the HAZ toughness, the minimum value was evaluated by the same method as in Example 1. However, the average value was determined from a Charpy impact test piece (V of JIS Z 2202) from a test piece prepared under the same conditions as in Example 1. Three notch specimens) were collected and evaluated by performing Charpy impact test at −20 ° C. and measuring absorbed energy vE- ₂₀ . From the collected Charpy impact test results for each three samples, the average value of absorbed energy vE- ₂₀ (ave) was determined, and the absorbed energy vE- ₂₀ (ave) exceeding 80 J was evaluated as having excellent HAZ toughness. .

  No. 1A, no. 6A, no. 26A, no. 26B is an invention example according to claim 3 in which chemical composition, oxide dispersion, and Ti-containing nitride dispersion are appropriately made, and the tensile strength and the minimum value of HAZ toughness are excellent, The average value of HAZ toughness exceeds 80 J, and it can be seen that the HAZ toughness and strength after large heat input are further excellent.

  In contrast, no. 1, no. 1B, No. 1 6, no. 6B, and no. Although 26 is a thick steel plate having excellent HAZ strength and toughness after large heat input, the dispersion of Ti-containing nitride does not satisfy the requirements of claim 3, so the average value of HAZ toughness is 80 J or less. It was.

Claims (6)

In mass%, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 1.4-3.0%, P: 0.03% or less (excluding 0%) ), S: 0.015% or less (excluding 0%), Al: 0.07% or less (including 0%), Cr: 0.5-2.0%, Ti: 0.010-0. 080%, REM: 0.0003-0.02%, Zr: 0.0003-0.02%, Ca: 0.0005-0.010%, N: 0.002-0.020%, A steel plate with the balance being iron and inevitable impurities,
The D value obtained from the equation D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr] satisfies 238 <D <388,
Constituent elements excluding oxygen contain oxides by mass% of 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50%, 5% <Zr <40% Of the oxides, 300 / mm ² or more of oxides having an equivalent circle diameter of less than 2 μm, 30 to 70 / mm ² of oxides having an equivalent circle diameter of 2 to 5 μm, and equivalent circle diameters. Is a thick steel plate excellent in toughness of the weld heat affected zone, characterized in that less than 30 oxides / mm ^{2 of} 5 μm or more exist.
However, in the above formula, [] indicates the content (% by mass) of each element. The constituent elements excluding oxygen are 10% by mass, 10% <Ti, Al <20%, 8% <Ca <40%, 5% <REM <50%, 5% <Zr <40%, and 10% <REM + Zr <70% is satisfied. Further, among oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4, 300 oxides / mm ² or more having an equivalent circle diameter of less than 2 μm The thick steel plate excellent in toughness of the weld heat affected zone according to claim 1, wherein the steel plate has excellent toughness. Furthermore, excellent toughness of the heat affected zone according to claim 1 or 2, characterized in that the circle equivalent diameter containing Ti-containing nitride of less than 0.05 .mu.m 5.0 × 10 ⁶ cells / mm ² or more Thick steel plate.   Furthermore, it contains at least one selected from the group consisting of Ni: 0.05 to 2.0%, Cu: 0.05 to 2.0%, and Mo: 0.05 to 2.0% by mass%. The thick steel plate excellent in the toughness of the welding heat affected zone according to any one of claims 1 to 3.   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 in the affected area.   Furthermore, it contains B: 0.0003-0.005% by mass%, The thick steel plate excellent in the toughness of the welding heat affected zone in any one of the Claims 1 thru | or 5 characterized by the above-mentioned.