JP2012162797A - Steel excellent in toughness of weld heat affected zone and method for producing thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel which has excellent HAZ toughness even when subjected to large heat input welding of a heat input of 50 kJ/mm or more and to provide a method for producing the same.SOLUTION: In the steel, (a) the composition of the entire oxide inclusions satisfies 5-50% ZrO, 5-50% REM oxides, and 50% or less (exclusive 0%) CaO when measured and expressed in terms of mass of a single oxide, (b) among the entire inclusions, the number of inclusions with a circle-equivalent diameter of 0.1-2 μm is 120 pieces/mmor more, the number of oxides with circle-equivalent diameters of above 3 μm is 5.0 pieces/mmor less, and the number of oxides with circle-equivalent diameters of above 5 μm is 5.0 pieces/mmor less. When the composition of the entire inclusions are measured, (c-1) the numerical ratio of the REM and Zr-containing inclusions I satisfying an REM/Zr mole ratio of 0.6-1.4 is 30% or more based on the number of the entire inclusions and/or (c-2) the numerical ratio of the REM, Zr, Al, Ca, and-Ti-containing inclusions II in which the ratio of the total number of moles of REM and Zr to the total number of moles of Al, Ca, and Ti satisfies 0.5-1.2 is 40% or more based on the number of the entire inclusions.

Description

  The present invention relates to a steel material used for a bridge, a high-rise building, a ship, and the like, and in particular, a part affected by heat when welding (hereinafter, referred to as “welding heat affected zone” or “HAZ”). .) Is related to a steel material excellent in toughness and a method for producing the same.

  The properties required for steel materials used in bridges, high-rise buildings, ships and the like have become increasingly severe in recent years, and particularly good toughness is required. Generally, these steel materials are often joined by welding, but among the welded joint portions, particularly HAZ has a problem that the toughness is easily deteriorated due to thermal influence during welding. This toughness deterioration becomes more prominent as the heat input during welding increases, and the cause is that the larger the heat input during welding, the slower the cooling rate of the HAZ, and the lower the hardenability and the generation of coarse island martensite. It is thought that there is to do. Therefore, in order to improve the toughness of the HAZ, it is considered that the heat input during welding should be suppressed as much as possible. However, on the other hand, in order to increase the welding work efficiency, it is desired to employ a high heat input welding method in which the heat input of welding is 50 kJ / mm or more, such as electrogas welding, electroslag welding, submerged arc welding, and the like.

Therefore, the present applicant has proposed steel materials that suppress the HAZ toughness deterioration when the high heat input welding method is adopted in Patent Documents 1 to 3. These steel materials are characterized in that they contain REM oxide and / or CaO and ZrO ₂ as oxides that become the core of intragranular ferrite transformation. Since the oxide exists in a liquid state in molten steel, it is finely dispersed in the steel. In addition, the oxide is thermally stable, and, for example, it does not dissolve and disappear even when exposed to a high temperature of 1400 ° C. for a long time, which greatly contributes to the improvement of HAZ toughness.

In addition, the present applicant has repeated research for providing a steel material excellent in HAZ toughness at the time of high heat input welding at a higher level even after disclosing the above-mentioned Patent Document 1, and as a result, described in Patent Document 4 The invention was proposed previously. In Patent Document 4, the size and number of all oxide inclusions in steel (not limited to oxides that become the core of intragranular ferrite transformation, but all oxides) are improved in HAZ toughness. In particular, if the number of coarse oxides with an equivalent circle diameter of more than 5.0 μm is reduced to 5 or less, the HAZ toughness can be improved even if large heat input welding with a heat input of about 50 kJ / mm is performed. It discloses that an excellent steel material can be obtained. Thus, according to Patent Document 4, since the number of coarse oxides is remarkably suppressed, even if welding is performed with a larger amount of heat input than the HAZ toughness evaluation method disclosed in the Example of Patent Document 1 above. The HAZ toughness could be increased. Specifically, in Patent Document 1, a heat cycle in which a temperature from 800 ° C. to 500 ° C. is cooled in 300 seconds after being held at a heating temperature of 1400 ° C. for 5 seconds (heat input condition: 1400 ° C. × 5 seconds, cooling Time Tc = 300 seconds) was given, and the absorbed energy (vE _-40 ) at −40 ° C. was measured. However, in Patent Document 4, the heat cycle (heat input condition: 1400 ° C.) in which the holding time at 1400 ° C. was increased to 30 seconds. X30 seconds, cooling time Tc = 300 seconds) was measured in the same manner as described above, and it was confirmed that good HAZ toughness was obtained even in this case.

On the other hand, Patent Documents 5 to 7 are not a technique in which an oxide of REM and ZrO ₂ are used in combination as in Patent Documents 1 to 4, but if REM is added to molten steel in which the amount of dissolved oxygen is adjusted, approximately It is disclosed that the HAZ toughness can be improved when high heat input welding exceeding 300 kJ / cm (about 30 kJ / mm) is performed.

Japanese Patent Laid-Open No. 2007-1001000 JP 2007-247004 A JP 2007-247005 A JP 2009-197267 A JP 2003-221463 A JP 2003-286540 A JP 2002-363687 A

  The present invention has been made by paying attention to the above-described circumstances, and the purpose thereof is a steel material excellent in HAZ toughness even when high heat input welding is performed with a heat input amount of 50 kJ / mm or more. And providing a manufacturing method thereof.

The steel material excellent in the toughness of the weld heat affected zone according to the present invention that has solved the above problems is C: 0.02 to 0.15% (meaning mass%; the same applies to the following components), Si: 0. 0.5% or less (not including 0%), Mn: 2.5% or less (not including 0%), P: 0.03% or less (not including 0%), S: 0.02% or less ( 0% not included), Al: not more than 0.050% (not including 0%), N: not more than 0.010% (not including 0%), Ti: 0.005 to 0.10%, Zr: 0.0005 to 0.050%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, and O: 0.0005 to 0.010%, with the balance being iron and A steel material made of inevitable impurities. And (a) When the composition of all oxide inclusions contained in the steel material is measured and converted into a single oxide by mass, the average composition is ZrO ₂ : 5 to 50%, REM oxide (REM When represented by the symbol M, M ₂ O ₃ ): 5 to 50%, CaO: 50% or less (excluding 0%) is satisfied, and (b) of all the inclusions contained in the steel material, Inclusions with an equivalent diameter of 0.1 to 2 μm are 120 or more per 1 mm ^{2 of} observation field area, and oxides with an equivalent diameter of 3 or more than 3 μm are 5.0 or less per 1 mm ^{2 of} observation field area. The number of oxides exceeding 5 μm is 5.0 or less per 1 mm ^{2 of the} observation visual field area. (C-1) When the composition of all inclusions contained in the steel material is measured, the number of all inclusions is REM. And Zr satisfying a molar ratio (REM / Zr) of 0.6 to 1.4 The number ratio of inclusion I is 30% or more, and / or (c-2) When the composition of all inclusions contained in the steel material is measured, REM and Zr with respect to the number of all inclusions And the ratio [(REM + Zr) / (Al + Ca + Ti)] of the total number of moles of Al, Ca and Ti satisfying 0.5 to 1.2, including REM, Zr, Al, Ca and Ti The point is that the number ratio of the product II is 40% or more.

The steel material, as another element,
[1] Cu: 2% or less (not including 0%) and / or Ni: 3.5% or less (not including 0%),
[2] Cr: 3% or less (not including 0%) and / or Mo: 1% or less (not including 0%),
[3] Nb: 0.25% or less (not including 0%) and / or V: 0.1% or less (not including 0%),
[4] B: 0.005% or less (excluding 0%)
Etc. may be contained.

The steel of the present invention, when the addition of REM to molten steel having an adjusted dissolved oxygen content Q _Of the range of 0.0003 to 0.01 mass%, the addition amount Q _REM of dissolved oxygen Q _Of the REM of the molten steel When adding REM, Zr, Ti, Ca, and Al to the molten steel in which the dissolved oxygen amount Q _Of is adjusted to the above range, REM and Zr are added. When the a-group element, Ti, Ca, and Al are b-group elements, the element can be manufactured by satisfying the following (2) and / or the following (3).
2logQ _REM + 3logQ _Of ≤-12.00 (1)
(2) About the said a group element, REM and Zr are added simultaneously, or the other element is added within 5 minutes after adding one element among REM and Zr.
(3) The case where the b group element is added before and / or after the addition of the a group element, and the b group element is added before the addition of the a group element. T1 (minutes) from the start of addition of the first element to the start of addition of the first element among the a group elements, and when the b group element is added after the addition of the a group element, the a group element The time from the start of addition of the last element to the start of addition of the first element among the group b elements is t2 (minutes), and the sum of t1 and t2 is 3 minutes or more. (0 ≦ t1, 0 ≦ t2, where t1 and t2 are not 0.)

  According to the present invention, an oxide (an oxide containing Zr, REM, and Ca) serving as a nucleus of intragranular α transformation (α means ferrite or a mixed structure of ferrite and bainite; the same shall apply hereinafter). Inclusions that are generated in a predetermined amount, and contain a predetermined element in a specific relationship with respect to the size and number of inclusions and oxides (ie, particle size distribution) present in the steel, and the total number of inclusions Therefore, it is possible to provide a steel material having excellent HAZ toughness during high heat input welding. In particular, in the steel material of the present invention, it has been clarified that not only a predetermined amount or more of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm useful for improving HAZ toughness are present, but also adversely affecting the improvement of HAZ toughness. The number of both coarse oxides with an equivalent circle diameter of more than 3 μm and super coarse oxides with an equivalent circle diameter of more than 5 μm is significantly suppressed, and the number of inclusions is REM and Zr. The number ratio of REM and Zr-containing inclusions I and / or the ratio of the total number of moles of REM and Zr and the total number of moles of Al, Ca, and Ti satisfy a specific relationship. Since the number ratio of the REM, Zr, Al, Ca, and Ti-containing inclusions II is a predetermined amount or more, welding is performed with a larger heat input than the HAZ toughness evaluation method disclosed in the example of Patent Document 4 above. HAZ tough It can be increased.

FIG. 1 shows an example of the order of addition of elements when the b group element is added before and after the addition of the a group element. FIG. 2 is a graph showing the relationship between the value (Z value) on the left side of equation (1) defined in the present invention and the number of oxides with an equivalent circle diameter exceeding 3 μm per 1 mm ² observation field area. FIG. 3 is a graph showing the relationship between the number of oxides with an equivalent circle diameter exceeding 3 μm per 1 mm ² observation field area and the absorbed energy (vE ₋₄₀ ) at −40 ° C.

  The present invention improves the technique using the oxide that is the nucleus of the intragranular α transformation disclosed in Patent Documents 1 to 4 above, and obtains a steel material in which the HAZ toughness does not deteriorate even when welding is performed with a larger heat input. For technology.

  That is, the present inventors have advanced research in order to provide a steel material excellent in HAZ toughness at the time of high heat input welding even after proposing the above-mentioned Patent Document 4. As a result, “a heat cycle in which the temperature from 800 ° C. to 500 ° C. is cooled in 400 seconds after holding at the heating temperature of 1450 ° C. for 5 seconds”, which is a condition of a larger heat input than that of Patent Document 4 (heat input conditions: In order to provide a steel material excellent in HAZ toughness even when given 1450 ° C. × 5 seconds and cooling time Tc = 400 seconds, as in Patent Document 4, five oxides having a circle equivalent diameter exceeding 5.0 μm are provided. It is not enough to reduce below, and the number of oxides exceeding 3.0 μm, which has not been noticed in the past including Patent Document 4, is reduced, and the composition of all inclusions contained in the steel material is measured. The number ratio of REM and Zr-containing inclusions I satisfying the REM to Zr molar ratio (REM / Zr) of 0.6 to 1.4 with respect to the number of all inclusions contained in the steel material is 30. % Or more of REM and Zr REM, Zr, Al, Ca, and Ti-containing inclusions II in which the ratio of the number of moles to the total number of moles of Al, Ca, and Ti [(REM + Zr) / (Al + Ca + Ti)] satisfies 0.5 to 1.2 The present inventors have found that it is very important that the number ratio is 40% or more, thereby completing the present invention.

Thus, the characteristic part of the present invention is
(A) Increasing the number of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm useful for improving HAZ toughness (120 pieces / mm ² or more),
(A) The number of oxides having an equivalent circle diameter of more than 5 μm that adversely affects the improvement of HAZ toughness is reduced (5.0 pieces / mm ² or less).
(C) The number of oxides having an equivalent circle diameter of more than 3 μm, which was first found to have an adverse effect on the improvement of HAZ toughness in the present invention, was reduced (5.0 pieces / mm ² or less), and (d) REM and Zr satisfying a REM / Zr molar ratio (REM / Zr) of 0.6 to 1.4 with respect to the number of all inclusions contained in the steel when the composition of all inclusions is measured. The ratio of the number of inclusions I is 30% or more, or the ratio between the total number of moles of REM and Zr and the total number of moles of Al, Ca and Ti with respect to the number of all inclusions contained in the steel material [ The number ratio of REM, Zr, Al, Ca, and Ti-containing inclusions II satisfying (REM + Zr) / (Al + Ca + Ti)] of 0.5 to 1.2 is 40% or more.

  By providing such a characteristic portion, the HAZ toughness can be improved even when welding is performed with a larger heat input than that of Patent Document 4. That is, in relation to the above-mentioned Patent Document 4, in addition to the above (a) and (b), the characteristic part of the present invention exists where the above (c) and (d) are defined.

  Strictly speaking, the definition of the above (a) is different from the above-mentioned Patent Document 4, and in Patent Document 4, the fine number in the oxide is controlled for the oxide. In the present invention, the difference is that not only the oxide but also all inclusions present in the steel material are controlled, and the fine number in the inclusions is controlled. According to the examination results of the present inventors, in order to realize good HAZ toughness, an oxide having a large equivalent circle diameter (hereinafter, sometimes simply abbreviated as “particle size”) (in the present invention, It has been found that the contribution of both oxides> 3 μm and oxides> 5 μm is very large. And if this large oxide is controlled so as not to be generated, the small inclusions having a particle size of 0.1 to 2 μm are not limited to oxides, and the desired characteristics can be secured even if they are expanded to all inclusions. It can be done.

Moreover, in order to provide the requirement (c), it is not sufficient to control the amount of dissolved oxygen in the molten steel before the addition of REM, as in Patent Document 4 and Patent Documents 5 to 7 described above, It has also been found that it is extremely important to appropriately control the _REM addition amount Q _REM according to the dissolved oxygen amount Q _Of in the molten steel. Specifically, an amount of REM (Q _REM ) that satisfies the following formula (1) is added according to the dissolved oxygen amount Q _Of of the molten steel before REM addition. Thereby, the production | generation of the REM type | system | group oxide with a large particle size which has a bad influence on realization of the desired HAZ toughness can be suppressed. Details of the technical significance of the following formula (1) will be described later.
2logQ _REM + 3logQ _Of ≤-12.00 (1)

Furthermore, in order to satisfy the requirement for the number ratio of inclusion I among the requirements (d) above, it is necessary to pay attention to the order of addition of REM and Zr, and the interval time between addition of these elements, and dissolved oxygen When adding REM to molten steel with adjusted amount Q _Of , add REM and Zr at the same time, or add one element of REM and Zr and add other element within 5 minutes It became clear that it was important to control.

  In addition, in order to satisfy the requirement of the number of inclusions II among the requirements of (d) above, it is important to appropriately control the addition conditions of REM, Zr, Ti, Ca, and Al. Became clear. Specifically, the addition order of each group when REM, Zr, Ti, Ca, and Al are divided into a group (REM and Zr) and b group (Ti, Ca, and Al), and the addition interval between groups It is necessary to pay attention to time.

  Details of the technical significance of the requirement (d) will be described later.

  In this specification, in order to distinguish between the oxides that are the core of the intragranular α-transformation, that is, the oxides containing Zr, REM, and Ca, and all the oxides contained in the steel material, the former is used for convenience of explanation. Is called “Zr / REM / Ca oxide”, and the latter is called “total oxide inclusions”. The oxide includes a single oxide and a composite oxide in which inclusions other than the oxide (for example, sulfide, nitride, carbide, or a composite compound thereof) are combined. In addition, the essential components (Zr, REM, and Ca) constituting the Zr / REM / Ca-based oxide may be particularly referred to as “intragranular α-transformation nucleation elements”.

  Here, the Zr / REM / Ca-based oxide that is the starting point of the intragranular α transformation will be described. The Zr / REM / Ca-based oxide means an oxide containing a Zr oxide, a REM oxide, and a Ca oxide. The elements (intragranular α-transformation nucleation elements) constituting the Zr / REM / Ca-based oxide are Zr, REM, and Ca. In addition to these, for example, oxides such as Ti, Mn, Si, and Al It may contain forming elements and other steel components.

The existence form of the Zr / REM / Ca-based oxide is not particularly limited, and may be present as a single oxide containing an intragranular α-transformation nucleation element alone, or may be present as an intragranular α-transformation nucleation element. You may exist as complex oxide containing 2 or more types. Examples of the single oxide include ZrO _{2 for} Zr; CaO for Ca; and REM for REM represented by the symbol “M”, such as M ₂ O ₃ , M ₃ O ₅ , and MO ₂ . These oxides may exist in an aggregated state, or may exist in a form in which other compounds such as sulfides and nitrides are complex-deposited on the oxides.

The Zr / REM / Ca-based oxide preferably further contains an oxide of Ti. When Ti oxide further exists, intragranular α transformation is promoted, and the improvement of HAZ toughness is further enhanced. The oxide of Ti may exist as a single oxide (for example, Ti ₂ O ₃ , Ti ₃ O ₅ , TiO ₂ ), or contains at least one of Zr / REM / Ca oxides and Ti. It may exist in the form of a complex oxide.

  Further, the steel material of the present invention includes sulfides, nitrides, carbides, or composite compounds thereof in addition to the oxides described above, but in this specification, oxides, sulfides, Nitride, carbide, or composite compounds thereof are collectively referred to as “all inclusions”.

  Moreover, in this specification, among all oxide inclusions contained in the steel material, an oxide having an equivalent circle diameter of 0.1 to 2 μm is referred to as “fine oxide”, and an oxide having an equivalent circle diameter of more than 3 μm. In some cases, “coarse oxides” and oxides having an equivalent circle diameter of more than 5 μm are referred to as “supercoarse oxides”, respectively. In Patent Document 4, an oxide having an equivalent circle diameter of more than 5 μm is defined as “coarse oxide”. However, in this specification, an oxide having an equivalent circle diameter of more than 3 μm is defined as “coarse oxidation”. Things ".

In this specification, “steel material having excellent HAZ toughness of high heat input welding” means a heat cycle in which a steel material is held at 1450 ° C. for 5 seconds and then a temperature from 800 ° C. to 500 ° C. is cooled in 400 seconds ( Heat history) (heat input condition: 1450 ° C. × 5 seconds, cooling time Tc = 400 seconds), it means that the absorbed energy (vE ₋₄₀ ) at ₋₄₀ ° C. satisfies 130 J or more. The larger vE _-40 is better, and preferably vE _-40 is 150 J or more. The above heat cycle may be particularly referred to as “large heat input heat history”. The amount of heat input by this heat cycle is higher than the amount of heat input by the heat cycle described in Patent Document 1 or Patent Document 4, and in that sense, the “large heat input welding” of the present invention and the above Patent Document. 1 and the heat input level of “Large heat input welding” described in Patent Document 4 are different.

  In the present invention, the temperature of the heat cycle is set to 1450 ° C. The heat temperature of the HAZ particularly close to the weld metal (sometimes called a bond portion) exceeds 1400 ° C. and is approximately 1450 ° C. It is considered to become.

  Hereinafter, the requirements (a) to (c) constituting the present invention will be described in detail.

[(A) Average composition of oxide]
The steel material of the present invention has an average composition of ZrO ₂ : 5 to 50% when the composition of all oxide inclusions contained in the steel material is measured and converted to mass as a single oxide (total is 100%). REM oxides (representing M ₂ O _{3 when} REM is represented by the symbol M): 5 to 50%, CaO: 50% or less (not including 0%) are satisfied. It works effectively as a nucleus. Below the lower limit of each oxide, the amount of oxide that becomes the nucleus of intragranular α-transformation at the time of welding is insufficient, and the effect of improving HAZ toughness is not exhibited. On the other hand, when the upper limit value of each oxide is exceeded, the oxide becomes coarse, the number of fine oxides that effectively act as nuclei of intragranular α transformation decreases, and the HAZ toughness improving effect is not exhibited effectively.

The ZrO ₂ is 5% or more, preferably 8% or more, more preferably 10% or more. On the other hand, the upper limit is 50%, the preferred upper limit is 45%, and the more preferred upper limit is 40%.

The oxide of the REM is 5% or more, preferably 10% or more, more preferably 13% or more. On the other hand, the upper limit is 50%, the preferred upper limit is 45%, and the more preferred upper limit is 40%. In addition, when the REM is represented by the symbol M, the REM oxide exists in the form of M ₂ O ₃ , M ₃ O ₅ , MO _2, etc. in the steel material. _It means the amount when converted to ₂ O ₃ .

  The CaO effectively acts as a nucleus for intragranular α-transformation, but if included excessively, the intragranular α-transformation ability deteriorates. Further, if CaO is excessively contained, the nozzle used for casting is melted. Therefore, the upper limit is 50%, preferably 45% or less, more preferably 40% or less, and particularly preferably 30% or less. In order to effectively exhibit the above action, CaO is preferably contained in an amount of 3% or more. CaO is more preferably 5% or more, and still more preferably 10% or more.

The remaining components of the composition of all oxide inclusions are not particularly limited, and oxides of oxide-forming elements contained in the steel material of the present invention (for example, SiO ₂ , Al ₂ O ₃ , MnO, etc.) Can be mentioned.

  The composition of all oxide inclusions contained in the steel material is the oxidation observed in the observation field by observing the surface of the steel material with, for example, an electron probe X-ray micro analyzer (EPMA). It can be measured by quantitative analysis. Details of the measurement conditions will be described in the column of Examples described later.

[(B) Particle size distribution of all inclusions]
Next, the number and size of all the inclusions that characterize the present invention will be described. The steel material of the present invention is
(I) The number of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm is 120 or more per 1 mm ^{2 of the} observation visual field area.
(Ii) The number of coarse oxides having an equivalent circle diameter exceeding 3 μm is 5.0 or less per 1 mm ^{2 of the} observation visual field area, and
(Iii) Super coarse oxide having an equivalent circle diameter exceeding 5 μm satisfies all of 5.0 or less per 1 mm ^{2 of the} observation visual field area. In particular, the present invention has the greatest feature when both (ii) and (iii) are defined for an oxide having a large equivalent-circle diameter (particle diameter).

Here, satisfying both of the requirements (ii) and (iii) means that the number of oxides having a particle size of more than 3 μm and less than 5 μm is as small as 5.0 or less. is doing. That is, in order to ensure a very high HAZ toughness of vE _-40 ≧ 130 J even when receiving a large heat input heat history according to the present invention, the above-mentioned Patent Document 4 did not pay any attention to “particle size of 3 μm to 5 μm or less. It is extremely important to reduce the number of oxides in the above range, and when the number of oxides in the range cannot be controlled, the present inventors consider that the HAZ toughness deteriorates due to the origin of brittle fracture. The results revealed for the first time.

  Hereinafter, the technical significance of (ii) and (iii) will be described in detail with reference to Tables 5 and 6 below.

  No. in Table 5 below. 1-32 are examples which satisfy all the requirements prescribed | regulated by this invention. Considering the above (ii) and (iii), no. No. 1 to No. 2 having the largest number of oxides exceeding 5 μm. 5 (1.440), the number of oxides exceeding 3 μm is suppressed to 4.64, and as a result, good HAZ toughness can be secured.

  On the other hand, no. 35 to 38, 49, 53, 54 and 61 are examples that satisfy the requirement (iii) but do not satisfy the requirement (ii). Specifically, the number of oxides exceeding 5 μm is limited to 0.440 to 2.250 and 5.0 or less, but the number of oxides exceeding 3 μm exceeds 5.0 and is from 5.71 to 10 As a result, the desired HAZ toughness was not obtained.

  Here, in the above No. 35 to 38, 49, 53, 54, and 61 are included in the range of Patent Document 4 in that they satisfy the requirement (iii), but are included in the range of Patent Document 4. Even if it does not satisfy | fill the requirements of said (ii), it turns out that the desired HAZ toughness prescribed | regulated by this invention cannot be achieved. Therefore, in the present invention, in addition to the above (iii), the above (ii) is further defined as a requirement for ensuring the desired HAZ toughness.

  Further, from the requirements (ii) and (iii), it can be seen that the number of oxides of more than 3 μm and not more than 5 μm is deeply involved in achieving the desired HAZ toughness. That is, depending on the manufacturing conditions, there may be more than 5.0 oxides in a very narrow range of more than 3 μm and less than 5 μm. Even if the number of fine regions in (i) is increased, Even if the number of super coarse regions in (iii) is reduced, the desired HAZ toughness cannot be obtained only by the presence of more than 5.0 oxides in the coarse regions of more than 3 μm and 5 μm or less. It was an unexpected finding for the people.

  The detailed mechanism is unknown as to why the desired HAZ toughness can be ensured by satisfying both (ii) and (iii) above, but the disappearance of TiN accelerates when the temperature exceeds 1400 ° C. and reaches 1450 ° C. Toughness decreases. However, it is considered that such a decrease in toughness can be suppressed by reducing the oxide of more than 3 μm and not more than 5 μm.

  As described above, in the present invention, it is necessary to satisfy the requirements (ii) and (iii) at the same time. That is, the number of coarse oxides having a particle size exceeding 3 μm is set to 5.0 or less, and the number of super coarse oxides having a particle size exceeding 5 μm is set to 5.0 or less. The smaller the number, the better. In any case, the number is preferably 3.0 or less, more preferably 2.0 or less, particularly preferably 1.0 or less, and most preferably 0. Specifically, it is preferable to control appropriately including the balance between the two, and within the scope of the present invention (both are 5.0 or less), the particle size is 5 μm than the coarse oxide having a particle size of more than 3 μm. It is preferable to reduce the number of super super coarse oxides. Specifically, the number of super coarse oxides is better as it approaches the lower limit (0), and is generally preferably 1.0 or less, and is most preferably close to 0 as opposed to coarse oxides. May be close to the upper limit (5.0), and is generally preferably 4.0 or less.

  The number of oxides with a circle equivalent diameter exceeding 3 μm and the number of oxides exceeding 5 μm are obtained by observing the cross section of the steel material with, for example, EPMA and quantitatively analyzing the component composition of inclusions observed in the observation field. The inclusion having an oxygen content of 5% or more is used as an oxide, and the equivalent circle diameter of the oxide may be obtained by observing and measuring with, for example, a scanning electron microscope (SEM).

  The above (ii) and (iii) that characterize the present invention have been described in detail.

In the steel material of the present invention, as defined in (i) above, it is necessary to provide 120 or more fine inclusions with an equivalent circle diameter of 0.1 to 2 μm per 1 mm ² of the observation visual field area. The number of fine inclusions is 120 or more per 1 mm ² of the observation visual field area, preferably 200 or more per 1 mm ² , more preferably 500 or more per 1 mm ^{2, and} still more preferably 700 or more per 1 mm ² .

  The number of fine inclusions having a circle equivalent diameter of 0.1 to 2 μm may be obtained by observing and measuring the cross section of the steel material with, for example, an SEM.

  In the steel material of the present invention, inclusions having an equivalent circle diameter of less than 0.1 μm are not included in the number of inclusions because they hardly contribute to the HAZ toughness improving effect by inclusion dispersion.

  The “equivalent circle diameter” is a diameter of a circle assumed to have the same area of inclusions (including oxides) and is recognized on the SEM observation surface.

[(C) Number ratio of REM and Zr-containing inclusion I satisfying REM / Zr ratio of 0.6 to 1.4, and (REM + Zr) / (Al + Ca + Ti) ratio satisfying 0.5 to 1.2 Number ratio of inclusions II containing REM, Zr, Al, Ca, and Ti]
In addition to the number and size of all inclusions being appropriately adjusted, the steel material of the present invention, when measuring the composition of all inclusions contained in the steel material, with respect to the number of all inclusions,
(C-1) Number ratio of REM and Zr-containing inclusion I (hereinafter, simply referred to as inclusion I) in which the molar ratio of REM to Zr (REM / Zr) satisfies 0.6 to 1.4 Is 30% or more,
(C-2) REM, Zr, wherein the ratio [(REM + Zr) / (Al + Ca + Ti)] of the total number of moles of REM and Zr and the total number of moles of Al, Ca and Ti satisfies 0.5 to 1.2 When the number ratio of Al, Ca, and Ti-containing inclusions II (hereinafter simply referred to as inclusions II) is 40% or more, the HAZ toughness is further improved.

  The above requirements (c-1) and (c-2) only need to satisfy at least one of them, and of course may satisfy both.

  The requirement (c-1) above is that REM / in order to realize desired HAZ toughness for inclusions containing REM and Zr among intragranular α-transformation nucleation elements (REM, Zr, and Ca). The molar ratio of Zr and the number ratio of the inclusion I are specified. On the other hand, the requirement (c-2) above is desirable for inclusions containing intragranular α-transformation nucleation elements (Zr, REM, and Ca) and other elements (Ti and Al) that constitute inclusions. The molar ratio of (REM + Zr) / (Al + Ca + Ti) and the number ratio of the inclusion II are specified in order to realize the HAZ toughness.

That is, as will be apparent from the examples described later, it has been found that even if the requirements (a) and (b) are substantially the same, the toughness value of the steel material varies. That is, as defined in the above (a) and (b), by controlling the average composition of oxides and the size and particle size distribution of inclusions, the absorbed energy at −40 ° C. even when welding with a large heat input is performed. (VE _-40 ) can achieve 100 J or more, but in addition to the above (a) and (b), the number ratio of inclusion I defined in (c-1) and / or the above (c-2) By controlling the number ratio of inclusions II, vE- ₄₀ can be 130 J or more.

Regarding the above (c-1), for example, No. shown in Table 5 below. 2 and No. 2 shown in Table 6 below. No. 33 had a difference of 42 J in the absorbed energy (vE _-40 ) at -40 ° C, though the particle size distribution of all the inclusions in (b) was almost the same. Therefore, as a result of further studies by the present inventors, regarding the REM and Zr constituting the inclusion, all of the inclusion I satisfying the molar ratio of REM to Zr (REM / Zr) satisfying 0.6 to 1.4. The number of inclusions I in which the number ratio to inclusions is controlled to 30% or more (above No. 2) is excellent in intragranular α-transformation ability and HAZ toughness, but satisfies the above ratio. It was found that the desired HAZ toughness could not be secured when the proportion was less than 30% (No. 33 above). REM and Zr are elements that generate an oxide that becomes the nucleus of intragranular α-transformation, and the relationship between the number ratio of inclusion I to all inclusions and HAZ toughness has a good correlation. The requirement (c-1) was specified. That is, when compared with inclusions having a REM / Zr ratio of less than 0.6 or a REM / Zr ratio of more than 1.4, the inclusion I having a REM / Zr ratio of 0.6 to 1.4 is satisfied. Is excellent in the intragranular α-transformation ability, and it has been found that this contributes to further refinement of the metal structure in HAZ and improvement of HAZ toughness.

And by making the number ratio of the inclusion I to the total number of inclusions 30% or more, the absorbed energy (vE- ₄₀ ) at -40 ° C can be 130J or more even when welding is performed with a large heat input. . The number ratio of the inclusion I to the total number of inclusions is preferably as large as possible, preferably 40% or more, more preferably 50% or more. The greater the number ratio of the inclusion I, the better, and most preferably 100%.

The above (c-2) is the same as the above (c-1). 17 and No. shown in Table 6 below. No. 51 had a difference of 36 J in the absorbed energy (vE _-40 ) at -40 ° C, although the particle size distribution of all the inclusions in (b) was almost the same. Therefore, as a result of further studies by the present inventors, regarding REM, Zr, Ti, Ca, and Al constituting the inclusion, the total number of moles of REM and Zr and the total number of moles of Al, Ca, and Ti In the case where the number ratio of inclusions II with respect to all inclusions satisfying the ratio [(REM + Zr) / (Al + Ca + Ti)] of 0.5 to 1.2 is controlled to 40% or more (the above No. 17) Although the α transformation is promoted and the HAZ toughness is improved, it has been found that the desired HAZ toughness cannot be secured if the number ratio of inclusions II satisfying the above ratio is less than 40% (the above No. 51).

  Inclusions II in which the (REM + Zr) / (Al + Ca + Ti) ratio satisfies the above range include REM and Zr among other elements (Al, Ca, Since it is appropriately controlled in relation to Ti), intragranular α transformation is promoted, and the metal structure in HAZ is further refined, so that HAZ toughness is improved.

  That is, when the relationship between the component composition of inclusions dispersed in steel and the HAZ toughness was examined, in order to refine the metal structure by generating intragranular α in HAZ, the core of intragranular α transformation The inclusion itself must have good consistency with the α phase. As an inclusion having good consistency with the α phase, it has been clarified by experiments by the present inventors that inclusions containing Ti in addition to REM and Zr are effective. However, in order for the intragranular α produced from the inclusion containing REM, Zr, and Ti to grow in the subsequent austenite phase, the inclusion itself containing REM, Zr, and Ti and the austenite phase It is desired that the consistency is also good. Therefore, the present inventors further studied to improve the consistency between the inclusion containing REM, Zr, and Ti and the austenite phase. If the melting point of the inclusion is controlled, the intragranular α transformation is performed. The knowledge that it can control was obtained. That is, once the inclusions melt in the austenite phase during welding, the affinity between the melted inclusions and the austenite phase becomes good, and the inclusions crystallize while maintaining consistency with the surrounding austenite phase during the cooling process. Is done. As the temperature further decreases, α phase begins to form, but it is preferentially generated from inclusions with good consistency with α, and α generated from inclusions has good consistency with austenite. The α transformation is promoted, and the effect of improving the HAZ toughness due to the refinement of the crystal is enjoyed.

  Therefore, the present inventors investigated the melting point behavior of inclusions using a high-temperature laser microscope in order to find a component composition region in which the melting point of the inclusions containing REM, Zr, and Ti decreases. As a result, the melting point of the inclusion containing REM, Zr, and Ti is affected by the contents of Ca and Al, and the (REM + Zr) / (Al + Ca + Ti) ratio based on the number of moles of these elements is 0.5. When it was in the range of -1.2, it was found that the melting point of inclusions was locally lowered and the intragranular α-transformation ability was increased.

  That is, when (REM + Zr) / (Al + Ca + Ti) ratio is less than 0.5 or (REM + Zr) / (Al + Ca + Ti) ratio exceeds 1.2, (REM + Zr) / (Al + Ca + Ti) is 0.5 to It was found that inclusion II satisfying 1.2 is excellent in the intragranular α-transformation ability and thus contributes to further refinement of the metal structure in HAZ and improvement of HAZ toughness.

  The composition of inclusions contained in the steel material may be obtained by observing the cross section of the steel material by, for example, EPMA and quantitatively analyzing the component composition of inclusions observed in the observation field. After measuring the composition, the number ratio of the inclusion I and the number ratio of the inclusion II in the total number of inclusions may be obtained. In addition, in the steel material of this invention, the composition is quantitatively analyzed about the inclusion whose circle equivalent diameter is 0.1 micrometer or more. This is because inclusions having an equivalent circle diameter of less than 0.1 μm are too small to be quantitatively analyzed with high accuracy.

  Next, the component composition in the steel material (base material) of the present invention will be described. In the steel material of the present invention, C: 0.02 to 0.15%, Si: 0.5% or less (not including 0%), Mn: 2.5% or less (not including 0%) as basic components , P: 0.03% or less (not including 0%), S: 0.02% or less (not including 0%), Al: 0.050% or less (not including 0%), N: 0.0. 010% or less (excluding 0%), Ti: 0.005 to 0.10%, Zr: 0.0005 to 0.050%, REM: 0.0003 to 0.015%, and Ca: 0.0003 -0.010% is contained. The reasons for setting these ranges are as follows.

  C is an element indispensable for securing the strength of the steel material (base material), and needs to be contained by 0.02% or more. The C amount is preferably 0.04% or more, more preferably 0.05% or more. However, if the amount of C exceeds 0.15%, a large amount of island martensite (MA) is generated in the HAZ at the time of welding and not only causes deterioration of the toughness of the HAZ, but also adversely affects the weldability. Therefore, the C content is 0.15% or less, preferably 0.10% or less, more preferably 0.08% or less.

  Si is an element that has a deoxidizing action and contributes to improving the strength of the steel (base material) by solid solution strengthening. In order to exhibit such an action effectively, Si is preferably contained in an amount of 0.01% or more. Si is preferably contained in an amount of 0.05% or more, more preferably 0.1% or more. However, if the Si content exceeds 0.5%, the weldability and toughness of the steel material deteriorate, so the Si content must be suppressed to 0.5% or less. The amount of Si is preferably 0.3% or less, more preferably 0.25% or less, and still more preferably 0.21% or less.

  Mn is an element that contributes to improving the strength of the steel material (base material). However, if the amount of Mn exceeds 2.5%, the weldability of the steel material (base material) is deteriorated. Therefore, the amount of Mn needs to be suppressed to 2.5% or less. The amount of Mn is preferably 2.30% or less, more preferably 2.0% or less. In addition, in order to exhibit the effect mentioned above effectively, it is preferable to contain Mn 0.2% or more. The amount of Mn is more preferably 0.40% or more, further preferably 0.60% or more, and particularly preferably 0.8% or more.

  P is an element that easily segregates, and particularly segregates at a grain boundary in a steel material to deteriorate the HAZ toughness. Therefore, the P amount needs to be suppressed to 0.03% or less. The P amount is preferably 0.02% or less, more preferably 0.015% or less. In general, P is unavoidably contained in an amount of about 0.001%.

  S is a harmful element that combines with Mn to produce sulfide (MnS) and degrades the toughness of the base material and the ductility in the thickness direction. Further, when S is combined with REM such as La or Ce to generate REM sulfide (for example, LaS or CeS), generation of oxide of REM is inhibited, so that HAZ toughness is deteriorated. Therefore, the S amount needs to be suppressed to 0.02% or less. The amount of S is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.006% or less. Note that S is usually unavoidably contained in an amount of about 0.0005%.

  Al is an element that acts as a deoxidizer. However, if added in excess, the oxide is reduced to form a coarse Al oxide, and the HAZ toughness deteriorates. Therefore, the Al amount must be suppressed to 0.050% or less. The Al content is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.025% or less, and particularly preferably 0.010% or less. Al is usually unavoidably contained in an amount of about 0.0005%.

  N is an element that precipitates nitrides (for example, ZrN and TiN), and the nitrides prevent the austenite grains generated in the HAZ from being coarsened during welding by the pinning effect and cause intragranular α transformation. Promotes and contributes to the improvement of HAZ toughness. As N increases, nitrides are formed to promote the refinement of austenite grains, so that it effectively works to improve the toughness of HAZ. However, when the N amount exceeds 0.010%, the solid solution N amount increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, the N amount needs to be suppressed to 0.010% or less. The N amount is preferably 0.0090% or less, more preferably 0.008% or less. In addition, in order to exhibit the effect mentioned above effectively, it is preferable to contain N 0.003% or more. The N amount is more preferably 0.004% or more, and further preferably 0.005% or more.

  Ti is an element that contributes to the improvement of HAZ toughness by generating a nitride such as TiN or an oxide containing Ti in the steel material. In order to exert such effects, it is necessary to contain Ti by 0.005% or more. The amount of Ti is preferably 0.007% or more, more preferably 0.010% or more. However, if added excessively, the base metal itself is hardened by solid solution strengthening of Ti, leading to a decrease in HAZ toughness. Therefore, Ti should be suppressed to 0.10% or less. The Ti content is preferably 0.07% or less, more preferably 0.06% or less.

Zr is an element that generates a complex oxide containing Zr and contributes to the improvement of HAZ toughness. In order to exert such an effect, it is necessary to contain 0.0005% or more. The Zr amount is preferably 0.0015% or more, more preferably 0.0020% or more. However, when Zr is added excessively, a large amount of coarse Zr oxide (for example, ZrO ₂ ) is generated and the HAZ toughness is deteriorated. Therefore, the amount of Zr is suppressed to 0.050% or less. The amount of Zr is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.01% or less.

  REM (rare earth element) and Ca are elements necessary to form respective oxides. By containing these oxides, it becomes easy to finely disperse the oxides, and the finely dispersed oxides become the nucleus of intragranular α transformation, which contributes to the improvement of HAZ toughness.

  REM should be contained by 0.0003% or more, preferably 0.001% or more, more preferably 0.0020% or more. However, when REM is added excessively, solid solution REM is generated and segregates to deteriorate the toughness of the base material. Therefore, the amount of REM should be suppressed to 0.015% or less. The REM amount is preferably 0.010% or less, more preferably 0.007% or less. In the present invention, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.

  Ca should be contained in an amount of 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more, and still more preferably 0.001% or more. However, when Ca is added excessively, coarse Ca sulfide is generated and the toughness of the base material deteriorates. Further, when Ca is added excessively, CaO is excessively generated and inclusions with a high CaO concentration are generated and deviates from the optimum inclusion composition range, so that the effect of acting as an intragranular transformation nucleus of inclusions is weakened. The HAZ toughness deteriorates instead. Therefore, the Ca content is suppressed to 0.010% or less. Ca is preferably 0.009% or less, more preferably 0.008% or less, and still more preferably 0.005% or less.

  The steel material of this invention contains the said element as an essential component, and O (oxygen) amount is 0.0005 to 0.010%. Here, the amount of oxygen indicates the total amount of oxygen, which means the total amount of oxygen forming oxides and free oxygen dissolved in the steel material. The remaining components of the steel material may be iron and inevitable impurities (for example, Mg, As, Se, etc.).

The steel material of the present invention is still another element,
[1] Cu: 2% or less (not including 0%) and / or Ni: 3.5% or less (not including 0%),
[2] Cr: 3% or less (not including 0%) and / or Mo: 1% or less (not including 0%),
[3] Nb: 0.25% or less (not including 0%) and / or V: 0.1% or less (not including 0%),
[4] B: 0.005% or less (excluding 0%),
It is also effective to contain such elements. The reasons for setting these ranges are as follows.

<< [1] Cu and / or Ni >>
Cu and Ni are both elements that contribute to increasing the strength of the steel material, and can be added alone or in combination.

  However, if the amount of Cu exceeds 2%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness is also lowered. Accordingly, the Cu content is preferably 2% or less. The amount of Cu is more preferably 1.8% or less, still more preferably 1.5% or less. In addition, in order to exhibit the effect | action by Cu addition effectively, it is preferable to make it contain 0.05% or more. The amount of Cu is more preferably 0.1% or more, and still more preferably 0.20% or more.

  If the Ni content exceeds 3.5%, the strength of the base material is significantly increased and the toughness of the base material is deteriorated, as in the case of Cu, so that the HAZ toughness is also reduced. Accordingly, the Ni content is preferably 3.5% or less. The amount of Ni is more preferably 3.0% or less, still more preferably 2.5% or less. In order to effectively exhibit the effect of adding Ni, it is preferable to contain 0.05% or more. The amount of Ni is more preferably 0.1% or more, and still more preferably 0.2% or more.

<< [2] Cr and / or Mo >>
Cr and Mo are both elements that contribute to increasing the strength of the steel material, and can be added alone or in combination.

  However, if Cr exceeds 3%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness is lowered. Therefore, the Cr content is preferably 3% or less. The amount of Cr is more preferably 2% or less, still more preferably 1.0% or less. In order to effectively exhibit the effect of addition of Cr, it is preferable to contain 0.05% or more. The amount of Cr is more preferably 0.1% or more, and still more preferably 0.15% or more.

  Similarly to Cr, when Mo exceeds 1%, the strength of the base material is significantly increased and the toughness of the base material is deteriorated, so that the HAZ toughness is lowered. Therefore, the Mo amount is preferably 1% or less. The amount of Mo is more preferably 0.9% or less, and still more preferably 0.8% or less. In addition, in order to exhibit the effect | action by Mo addition effectively, it is preferable to make it contain 0.05% or more. The amount of Mo is more preferably 0.1% or more, and further preferably 0.15% or more.

<< [3] Nb and / or V >>
Nb and V are elements having an action of precipitating as carbonitride and preventing the austenite grains from coarsening during welding and improving the HAZ toughness due to the pinning effect of the carbonitride. Nb and V can be added alone or in combination.

  However, if the Nb content exceeds 0.25%, the precipitated carbonitrides become coarse and deteriorate the HAZ toughness. Accordingly, the Nb content is preferably 0.25% or less. The amount of Nb is more preferably 0.2% or less, still more preferably 0.15% or less. In order to effectively exhibit the effect of Nb addition, it is preferable to contain 0.002% or more. The amount of Nb is more preferably 0.010% or more, and further preferably 0.02% or more.

  If V exceeds 0.1% as in Nb, the precipitated carbonitrides become coarse and deteriorate the HAZ toughness. Therefore, the V amount is preferably 0.1% or less. The amount of V is more preferably 0.09% or less, still more preferably 0.08% or less. In order to effectively exhibit the effect of V addition, it is preferable to contain 0.002% or more. V amount is more preferably 0.005% or more, and still more preferably 0.01% or more.

<< [4] B (boron) >>
B is an element that suppresses the formation of grain boundary ferrite and improves toughness. However, if the amount of B exceeds 0.005%, it precipitates as BN at the austenite grain boundary, leading to a decrease in toughness. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.0030% or less. In addition, in order to exhibit the effect | action by B addition effectively, it is preferable to make it contain 0.001% or more. The amount of B is more preferably 0.0015% or more.

  Next, a manufacturing method that can be suitably employed in manufacturing the steel material of the present invention will be described.

To produce the steel material of the present invention,
(1) Upon the addition of REM to molten steel having an adjusted dissolved oxygen content Q _Of the range of 0.0003 to 0.01 mass%, the addition amount Q _REM of dissolved oxygen Q _Of the REM of the molten steel below (1 It is necessary to add an amount of REM that satisfies the formula.
2logQ _REM + 3logQ _Of ≤-12.00 (1)

In addition, when adding REM, Zr, Ti, Ca, and Al to the molten steel with the dissolved oxygen amount Q _Of adjusted to the above range, REM and Zr are group a elements, and Ti, Ca, and Al are group b elements. In addition, it is also important that the addition conditions of each element satisfy the following (2) and / or the following (3).
(2) About the said a group element, REM and Zr are added simultaneously, or the other element is added within 5 minutes after adding one element among REM and Zr.
(3) The case where the b group element is added before and / or after the addition of the a group element, and the b group element is added before the addition of the a group element. T1 (minutes) from the start of addition of the first element to the start of addition of the first element among the a group elements, and when the b group element is added after the addition of the a group element, the a group element The time from the start of addition of the last element to the start of addition of the first element among the group b elements is t2 (minutes), and the sum of t1 and t2 is 3 minutes or more. (0 ≦ t1, 0 ≦ t2, where t1 and t2 are not 0.)
Details will be described below.

[(1) Relationship between dissolved oxygen content of molten steel and added amount of REM]
The above formula (1) is set in order to ensure the desired HAZ toughness defined in the present invention. Based on the above formula (1), the amount of REM added according to the dissolved oxygen amount Q _Of of the molten steel Appropriate addition of Q _REM can ensure the desired HAZ toughness (see Examples below).

In addition, the coefficient on the left side of the above formula (1) is a value based on the REM oxide generation reaction formula in the molten steel represented by the following formula (2).
2REM + 3O = REM ₂ O ₃ (2)

The fact that the dissolved oxygen amount Q _{Of of the} molten steel and the REM addition amount Q _REM satisfy the above equation (1) means that the REM addition amount Q _REM involved in the generation of the REM oxide is set to be small. . As a result, the number of oxides of REM to be generated is reduced, and as a result, the number of coarse and super coarse oxides is reduced within the scope of the present invention, and the desired HAZ toughness is ensured. It is thought to be.

When the Z value exceeds -12.00, the balance between the dissolved oxygen amount Q _Of of the molten steel and the REM addition amount Q _REM becomes poor, and the REM addition amount Q _REM increases, resulting in the formation of coarse REM oxides. To do. As a result, the HAZ toughness decreases. Therefore, the Z value is set to -12.00 or less. Z value becomes like this. Preferably it is -12.25 or less, More preferably, it is -12.50 or less, More preferably, it is -12.75 or less. The lower limit of the Z value is not particularly limited, but is generally about −15 in consideration of the amount of REM in the steel.

In Patent Document 4, no consideration is given to the above formula (1). Therefore, there is a case that a lot of amount Q _REM of REM (1) does not satisfy the relationship of expression, as (1) of the value of the left-hand side (Z value) exceeds -12.00. Further, although Patent Documents 5 to 7 described above describe adding REM to molten steel in which the dissolved oxygen amount Q _Of is adjusted, the REM addition amount Q _REM is determined according to the dissolved oxygen amount Q _Of. The points to be added are not considered at all. Moreover, in the said patent documents 5-7, since it is not described about using REM, Zr, and Ca together, it contains Zr, REM, and Ca which have a HAZ toughness improvement effect | action as prescribed | regulated by this invention. An oxide (Zr / REM / Ca-based oxide) has not been obtained in the first place.

Next, the REM addition amount Q _REM and the dissolved oxygen amount Q _Of constituting the equation (1) will be described.

First, the REM addition amount Q _REM may be appropriately added according to the dissolved oxygen amount Q _Of as described above. The amount of REM added Q _REM is set larger than the amount of REM contained in the steel material of the present invention. This is because the amount of REM added before casting is volatilized in the casting process or dispersed in the slag, and the amount of REM contained in the steel material is reduced.

Further, the dissolved oxygen amount Q _Of of the molten steel is set to a range of 0.0003 to 0.01% by mass. Dissolved oxygen means oxygen in a free state that does not form an oxide and exists in molten steel. That is, in order to manufacture the steel material of the present invention, first, as a precondition, the dissolved oxygen amount Q _Of of the molten steel is adjusted to a range of 0.0003 to 0.01% by mass. If the dissolved oxygen amount Q _Of of the molten steel is less than 0.0003 mass%, the dissolved oxygen amount Q _Of of the molten steel is insufficient, so that a predetermined amount of Zr, REM, and Ca-based oxides that are the core of intragranular α transformation cannot be secured. , HAZ toughness cannot be improved. Further, when the dissolved oxygen amount Q _Of is insufficient, Zr, which could not form an oxide, forms a carbide, or REM or Ca forms a sulfide, which causes the toughness of the base material itself to deteriorate. Therefore, the dissolved oxygen amount Q _Of is set to 0.0003 mass% or more. The dissolved oxygen amount Q _Of is preferably 0.001% by mass or more, more preferably 0.0020% by mass or more.

On the other hand, when the amount of dissolved oxygen Q _Of exceeds 0.01% by mass, the amount of dissolved oxygen in the molten steel is too large, and the reaction between oxygen in the molten steel and the above elements becomes violent, which is not preferable for the melting operation. A coarse oxide or a super coarse oxide is produced to deteriorate the HAZ toughness. Therefore, the dissolved oxygen amount Q _Of should be suppressed to 0.01% by mass or less. The dissolved oxygen amount Q _Of is preferably 0.008% by mass or less, more preferably 0.007% by mass or less.

By the way, the dissolved oxygen amount Q _Of in the molten steel primarily refined in the converter or electric furnace usually exceeds 0.01% by mass. Therefore, in the production method of the present invention, it is necessary to adjust the dissolved oxygen amount Q _Of of the molten steel to the above range by some method.

Examples of the method for adjusting the dissolved oxygen amount Q _Of of the molten steel include a method of vacuum deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, Ti, Al and the like. The amount of dissolved oxygen Q _Of may be adjusted by appropriately combining these methods. Further, the dissolved oxygen amount Q _Of may be adjusted using a ladle heating type refining device or a simple molten steel processing facility instead of the RH type degassing refining device. In this case, since the amount of dissolved oxygen Q _Of cannot be adjusted by vacuum deoxidation, a method of adding a deacidifying element such as Si may be adopted to adjust the amount of dissolved oxygen Q _Of . When employing a method of adding a deoxidizing element such as Si, the deoxidizing element may be added when steel is removed from the converter to the ladle.

[Addition order of (2, 3) REM, Zr, Ti, Ca, and Al]
After adjusting the dissolved oxygen amount Q _Of of the molten steel to the above range as described above, the number ratio of the inclusion I is set to 30% or more as defined in the above (c-1). It is important that the addition condition of Zr satisfies the requirement (2) above. To make the number ratio of the inclusions II 40% or more as defined in the above (c-2), REM, It is important that the addition conditions of Zr, Ti, Ca, and Al satisfy the requirement (3). Therefore, the order of addition is only required to satisfy at least one of them, but in order to set the number ratio of inclusions I to 30% or more and the number ratio of inclusions II to 40% or more, (2 It is important to satisfy both requirements) and (3).

[(2) Order of addition of REM and Zr]
In (2), only the order of addition of the group a elements (REM, Zr) is defined, whereby the number ratio of inclusions I can be adjusted.

In order to increase the number ratio of the inclusion I to the total number of inclusions, it is necessary to add REM and Zr simultaneously or almost simultaneously (within 5 minutes) to the molten steel with the dissolved oxygen amount Q _Of adjusted. There is.

  When adding REM and Zr separately, Zr may be added after adding REM, or REM may be added after adding Zr. In either case, REM (or Zr) is added. It is necessary to set the interval from the addition to the addition of Zr (or REM) within 5 minutes. This interval is preferably within 4 minutes, more preferably within 3 minutes.

  Note that it is preferable to pay attention to the order of addition of the b group element (Ti, Ca, Al) for the purpose of further improving the HAZ toughness by the Zr / REM / Ca-based oxide. For example, Ca is recommended to be added after REM and Zr.

  Moreover, for the purpose of further improving the HAZ toughness due to the refinement of Ti oxide, for example, Ti is preferably added to the molten steel before REM is added. Ti oxide has a smaller interfacial energy with molten steel than Zr, REM, and Ca-based oxides, so adding Ti before adding Zr, REM, and Ca to the molten steel makes the Ti oxide finer. As a result, fine oxides contributing to HAZ toughness can be generated. Then, after adding Ti, Zr, REM, and Ca are added as described above to obtain a Zr / REM / Ca-based oxide that becomes the nucleus of the desired intragranular α-transformation.

Even when REM is added after adding Ti to the molten steel with the dissolved oxygen amount Q _Of adjusted, the REM addition amount Q _REM is expressed by the above equation (1) according to the dissolved oxygen amount Q _Of of the molten steel, as will be described later. If REM is added so as to satisfy the above, the size and density of the oxide can be appropriately controlled. When Ti is added prior to REM, the dissolved oxygen in the molten steel is reduced by bonding with Ti to form an oxide, but Ti is less likely to bond with oxygen than REM, and Ti oxide is an interface with the molten steel. This is because the energy is small and it is difficult to form a coarse oxide having an equivalent circle diameter exceeding 3 μm. In addition, since REM and Zr are more easily bonded to oxygen than Ti, the inclusions can be generated even if Ti is added before REM and Zr.

[(3) Order of addition of group a elements (REM, Zr) and group b elements (Ti, Ca, and Al)]
(3) defines the addition conditions of the a-group element and the b-group element, whereby the number ratio of inclusions II can be adjusted.

In order to increase the number ratio of the inclusions II to the total number of inclusions, the addition conditions of REM, Zr, Ti, Ca, and Al added to the molten steel with adjusted dissolved oxygen amount Q _Of are appropriately controlled. There is a need to. Specifically, when REM and Zr are a group elements and Ti, Ca, and Al are b group elements, it is necessary to add the b group elements before and / or after the addition of the a group elements. That is, the a group element and the b group element should not be added at the same time but should be added with a time difference.

  Moreover, it is necessary to appropriately control the addition interval time of the a group element and the b group element. That is, in the case where the b group element is added before the addition of the a group element, the time from the start of addition of the first element of the b group elements to the start of addition of the first element of the a group elements is defined as t1 (minutes). ), In the case where the b group element is added after the addition of the a group element, the first element of the b group elements from the start of addition of the last element of the a group elements (the b group added first after the addition of the a group element) When the time until the start of the addition of (element) is t2 (minutes), the total of t1 and t2 needs to be 3 minutes or more.

  In calculating the t1, the time until the start of addition of the first element among the group a elements means the time until the first group a element is added. For example, when adding REM and Zr at the same time, it is the time until the simultaneous addition, and when adding REM after adding REM, REM (element added first among group a elements) is added. It means the time until the point of addition.

  In calculating t2, the time until the start of the addition of the last element among the a group elements means the last time when all the a group elements are added. For example, when REM and Zr are added at the same time, it is the time when they are added simultaneously. When Zr is added after REM is added, the time when Zr (the element added last among the group a elements) is added. Means the time until.

Here, the addition interval time of the a group element and the b group element, and the addition order of the a group element and the b group element will be described with reference to the drawings. FIG. 1 shows an example of the order of addition of elements when the b group element is added before and after the addition of the a group element. In FIG. 1, a ₁ and a ₂ indicate a group element, and ♦ indicates the start time of addition of each element. Further, b _{1 to} b ₄ represent b group elements, and ● represents the start of addition of each element.

In FIG. 1, elements are added in the order of b ₁ → b ₂ → a ₁ → a ₂ → b ₃ → b ₄ , and the first element added among the a group elements is a ₁ and the last of the a group elements. The element to be added is a ₂ , the first element added among b group elements is b ₁ , and the b group element added first after adding the a group element is b ₃ . The above t1, because the time from the start of addition time of the first element of the b-group element to the start of the addition time of the first element of a group element, FIG. 1, a ₂ start of the addition time of the b ₁ The time until the start of addition is t1. In addition, since t2 is the time from the start of addition of the last element of the a group element to the start of addition of the first element of the b group element, in FIG. 1, from the start of addition of a _{2 in} FIG. The time until the start of addition of b ₃ is t2.

In FIG. 1, when a group element is added simultaneously, a ₁ and a ₂ may be added simultaneously. In this case, the addition start time of the first element of the a group element and the last of the a group elements are added. The starting point of element addition is the same.

  The addition interval time of the group a element and the group b element will be described in more detail with a specific example.

  First, the case where it adds in order of Al-> Ti-> REM-> Zr-> Ca is demonstrated as a 1st example. In this case, t1 is the time from the start of addition of Al (b group element added first) to the start of addition of REM (a group element added first), and t2 is Zr. It means the time from the start of addition of (the last added a group element) to the start of addition of Ca (the first b group element added for the remaining b group elements).

  As a second example, a case where Al → Ti → REM and Zr are added in the order of simultaneous → Ca will be described. In this case, t1 is the time from the start of addition of Al (group b element added first) to the start of addition of REM and Zr, and t2 is from the start of addition of REM and Zr. It means the time until the start of addition of Ca (b group element added first for the remaining b group elements).

  The total of the above t1 and t2 is 3 minutes or more. By setting the total of t1 and t2 to 3 minutes or more, inclusions containing REM and Zr and containing appropriate amounts of Al, Ca, and Ti can be generated. The total of t1 and t2 is preferably 5 minutes or more, more preferably 7 minutes or more. The upper limit of the total of t1 and t2 is not particularly limited, but if the time is too long, the productivity is lowered, so the upper limit is about 20 minutes.

  When the a group element is not added before the b group element is added, or when the a group element is not added after the b group element is added, t1 or t2 may be calculated as 0 minutes. However, t1 = t2 = 0 is excluded.

  Regarding the addition order of the a group element and the b group element, the b group element may be added after the a group element is added, or the a group element may be added after the b group element is added. . Alternatively, the a-group element may be added after the b-group element is added, and then the b-group element may be added. When the b group element is added both before and after the addition of the a group element, the type and content of all the b group elements may be controlled before and after the addition of the a group element. For example, after adding a part of the b group element, the a group element may be added, and then the rest of the b group element may be added, or the same element may be added repeatedly before and after the a group element is added. May be.

  The a-group element and the b-group element may be added simultaneously within a range that satisfies the requirement (b-2), or may be added separately.

  Further, when adding the b group element, if the addition condition of REM and Zr satisfies the requirement (b-1), the number ratio of the inclusion I can be controlled to 30% or more, HAZ toughness can be improved.

  The form of REM, Ca, Zr, and Ti added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or pure Ca, pure Zr, pure Ti, and further Fe-Si. -La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Fe-Zr alloy, Fe-Ti alloy, Ni-Ca alloy, etc. That's fine. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, the range specified in the present invention must be satisfied.

  The molten steel obtained by adjusting the components in this manner may be continuously cast according to a conventional method to form a slab, and then hot rolled according to a conventional method.

When the steel material of the present invention is held at 1450 ° C. for 5 seconds and then has a heat history of cooling from 800 ° C. to 500 ° C. for 400 seconds (heat input condition: 1450 ° C. × 5 seconds, cooling time Tc = 400 seconds), it is possible to secure 130 J or more in absorbed energy (vE _-40 ) at -40 ° C. Therefore, the steel material according to the present invention can be used as a material for structures such as bridges, high-rise buildings, and ships, for example, in small to medium heat input welding as well as in large heat input welding with a heat input of 50 kJ / mm or more. It is possible to prevent toughness deterioration of the weld heat affected zone. The steel material of the present invention is intended for a thick steel plate having a thickness of about 3.0 mm or more.

  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, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Using a vacuum melting furnace (capacity 150 kg), under the conditions shown in Tables 1 and 2 below, the test steels with the composition (mass%) shown in Tables 3 and 4 below (the balance being iron and inevitable impurities) were melted Then, it was cast into a 150 kg ingot and cooled. Thereafter, heating and rolling were performed to produce a thick steel plate. Of the test steels shown in Tables 3 and 4 below, it was confirmed that the total O content of the test steels satisfying the requirements specified in the present invention was in the range of 0.0005 to 0.010%. Yes.

In melting the test steel in a vacuum melting furnace, the components other than Ti, Zr, REM, and Ca are adjusted, and at least one element selected from C, Si, Mn, and Al is used. Was used to adjust the dissolved oxygen amount Q _Of of the molten steel. Table 1 below shows the dissolved oxygen amount Q _Of after the adjustment.

After adding Ti to the molten steel in which the dissolved oxygen amount Q _Of was adjusted, Ca was added after adding Zr and REM. The order of addition of Zr and REM is shown in Tables 1 and 2 below. At this time, when Zr is added after adding REM, or when REM is added after adding Zr, the time (addition) required from adding one element to adding the other element Table 1 and Table 2 below show the (interval time). Tables 1 and 2 below show the total addition interval time (t1 + t2) of the group a elements (REM and Zr) and the group b elements (Ti, Ca, and Al).

Further, the amount of REM added is Q _REM, and this value is shown in Tables 1 and 2 below. Further, Z values calculated by substituting the values of the dissolved oxygen amount Q _Of and the REM addition amount Q _{REM into} the following equation (1) ′ are shown in Tables 1 and 2 below.
Z = 2logQ _REM + 3logQ _Of (1) '

  Ti is in the form of an Fe-Ti alloy, Zr is in the form of an Fe-Zr alloy, REM is in the form of a misch metal containing about 25% La and about 50% Ce, and Ca is a Ni-Ca alloy. Each was added in form. However, no. 12, No. 4 in Table 4 below. 39 and 41 were not in the form of misch metal, but only Ce was added.

  After adding the above elements, it was cast into an ingot and cooled. The obtained ingot was hot-rolled to produce a thick steel plate having a thickness of 30 to 80 mm. A sample was cut out from the cross section at t / 4 (where t is the thickness of the steel plate) of the resulting thick steel plate, the component composition of all oxide inclusions contained in the sample was measured, and the mass as a single oxide The average composition of the oxide was calculated in terms of conversion.

  The component composition of all oxide inclusions was measured by the following procedure. The cut sample surface was observed using an electron beam microprobe X-ray analyzer (EPMA; “JXA-8500F (device name)”) manufactured by JEOL Datum, and an inclusion with an equivalent circle diameter of 0.1 μm or more. The component composition was quantitatively analyzed. The observation conditions were an acceleration voltage of 20 kV, a sample current of 0.01 μA, and an analysis number of 100 or more, and the component composition at the center of the inclusion was quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays. The analysis target elements are Si, Mn, S, Al, Ti, Zr, La, Ce, Ca, and O (oxygen), and using a known substance, the relationship between the X-ray intensity and the element concentration of each element is pre-calibrated. The amount of elements contained in the inclusions was quantified from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve.

Of the obtained quantitative results, inclusions having an oxygen content of 5% by mass or more were defined as oxides. At this time, when a plurality of elements were observed from one inclusion, the oxide composition was calculated by converting the mass of each element into a single oxide from the ratio of X-ray intensity indicating the presence of these elements. . In the present invention, mass conversion is performed as the single oxide in this way, and the average is obtained as the average composition of the oxide. Of the oxides, the average compositions of ZrO ₂ , REM oxide, and CaO are shown in Tables 5 and 6 below. REM oxides, when the metal element is represented by M, exist in the form of M ₂ O ₃ , M ₃ O ₅ or MO _{2 in} the steel material, but all oxides are converted to M ₂ O ₃ The composition was calculated. “Others” shown in Tables 5 and 6 below are oxides other than ZrO ₂ , REM oxide, and CaO (for example, Al ₂ O ₃ , MnO, SiO _2, etc.).

Next, the circle-equivalent diameter of the quantified inclusions was measured by SEM observation, and the number of inclusions having a circle-equivalent diameter (particle diameter) of 0.1 to 2.0 μm was measured. Tables 5 and 6 below show the numbers obtained by converting the measurement results per 1 mm ² of the observation visual field area.

In addition, among the obtained quantitative results, the equivalent circle diameter of an oxide having an oxygen content of 5% by mass or more was measured by SEM observation, and the number of oxides having an equivalent circle diameter (particle diameter) exceeding 3 μm and the equivalent of the circle were measured. The number of oxides having a diameter (particle diameter) exceeding 5 μm was measured. Tables 5 and 6 below show values obtained by converting the number of oxides per 1 mm ² observation field area.

FIG. 2 shows the relationship between the Z value and the number of oxides with an equivalent circle diameter exceeding 3 μm per 1 mm ² observation field area. FIG. 2 shows the numbers shown in Tables 5 and 6 below. 1 to 32 (circles in FIG. 2) and No. Of the results of 35-40, 53, 54, 61 (circles in FIG. 2), in order to show the critical significance of the Z value, plot the Z value in the range of -12.50 to -11.50 did.

As is apparent from FIG. 2, if REM is added so as to satisfy the above equation (1) according to the dissolved oxygen amount Q _Of of the molten steel, the generation of oxides having an equivalent circle diameter exceeding 3 μm can be suppressed. I understand.

  Next, for the inclusions containing REM and Zr among the quantified inclusions, the molar ratio of REM and Zr is calculated, and the REM / Zr ratio is 0.6 to 1.4 with respect to the number of all inclusions. The number ratio of REM and Zr-containing inclusions I satisfying the above range (number ratio of inclusions I) was calculated, and the results are shown in Tables 5 and 6 below. Of the quantified inclusions, for inclusions containing REM, Zr, Ti, Ca, and Al, the ratio between the total number of moles of REM and Zr and the total number of moles of Al, Ca, and Ti [(REM + Zr) / ( Al + Ca + Ti)], and REM, Zr, Al, Ca, and Ti-containing inclusions satisfying the range of (REM + Zr) / (Al + Ca + Ti) ratio of 0.5 to 1.2 with respect to the number of all inclusions The number ratio of II (number ratio of inclusion II) was calculated, and the results are shown in Tables 5 and 6 below.

  Next, in order to evaluate the toughness of the HAZ that is affected by heat during welding, the following welding reproduction test was performed by simulating high heat input welding. In the welding reproduction test, the sample cut from the t / 4 position (where t is the plate thickness) of the thick steel plate was heated to 1450 ° C., held at this temperature for 5 seconds, and then given a heat cycle for cooling. . The cooling rate was adjusted so that the cooling time from 800 ° C. to 500 ° C. was 400 seconds (heat input condition: 1450 ° C. × 5 seconds, cooling time Tc = 400 seconds).

The impact characteristics of the sample after cooling were evaluated by taking three V-notch Charpy test pieces in the rolling direction from the sample after applying the thermal cycle and conducting an impact test according to JIS Z2242. In the impact test, the absorbed energy (vE _-40 ) at -40 ° C was measured, and the average value of three times was calculated. In the present invention, a sample having an average value of vE- ₄₀ of 130 J or more is considered acceptable (haz toughness is good). The measurement results are shown in Tables 5 and 6 below.

The following Table 1 to Table 6 can be considered as follows. No. 1-32 are examples that satisfy the requirements stipulated in the present invention. When the composition of all oxide inclusions contained in the steel material is measured and converted into a single oxide in mass, the oxidation of ZrO ₂ and REM And an oxide having an equivalent circle diameter of more than 3 μm and an oxide having an equivalent circle diameter of more than 5 μm, and an equivalent circle diameter of 0.1 to 2 μm. The number ratio of the inclusion I is 30% or more and / or the number ratio of the inclusion II is 40% or more with respect to the total number of inclusions. Therefore, a steel material having good HAZ toughness is obtained. It can also be seen that the higher the Si content, the better the HAZ toughness.

  On the other hand, no. 33 to 64 are examples that do not meet any of the requirements defined in the present invention. Of these, No. 33, 51, 52, 55-58, 62, 64 are the time from the addition of REM to the addition of Zr. In Nos. 34 and 60, since the time from the addition of Zr to the addition of REM does not satisfy the requirement defined in the present invention, the number ratio of inclusion I is less than 30%. Accordingly, the HAZ toughness is deteriorated.

No. 35 to 40, 53, 54 and 61 are oxides having an equivalent circle diameter exceeding 3 μm because the balance between the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _REM of the _REM does not satisfy the above formula (1). In particular, an oxide having an equivalent circle diameter of more than 3 μm and not more than 5 μm is generated. Accordingly, the HAZ toughness is deteriorated. No. 41, because the amount of oxide of REM when the composition of all oxide inclusions contained in the steel material is measured and converted into a single oxide by mass is less than the range defined in the present invention, The oxide amount serving as the nucleus of transformation is insufficient, and the HAZ toughness is deteriorated.

No. 42 and no. 59, the amount of REM contained in the steel material is large, and the amount of REM oxide when the composition of all oxide inclusions contained in the steel material is measured and converted into a single oxide by mass exceeds the range specified in the present invention. Therefore, the oxide becomes coarse, the number of fine oxides acting as nuclei for intragranular α transformation decreases, and the HAZ toughness improving effect is not exhibited. No. No. 43, because the amount of Zr contained in the steel material is too small, the amount of ZrO ₂ occupying the composition of all oxide inclusions is reduced, and the amount of Zr / REM / Ca oxides that become the core of intragranular α transformation is reduced. It is thought that. Therefore, the HAZ toughness is deteriorated. No. 44 and no. No. 63 has a large amount of ZrO _{2 in} the composition of all oxides because the amount of Zr contained in the steel material is too large. For this reason, the amount of oxide that becomes the nucleus of intragranular α-transformation is insufficient at the time of welding, and a microstructure cannot be obtained, and the HAZ toughness is deteriorated.

  No. In No. 45, since the amount of Ca contained in the steel material is too large, the amount of CaO in the composition of all oxide inclusions is large. For this reason, the amount of oxide that becomes the nucleus of intragranular α-transformation is insufficient at the time of welding, and a microstructure cannot be obtained, and the HAZ toughness is deteriorated. No. Since the amount of Ca contained in the steel material 46 is too small, the amount of CaO is not generated. Therefore, the amount of Zr / REM / Ca-based oxide that becomes the nucleus of the intragranular α transformation is reduced, and the HAZ toughness is deteriorated. No. In No. 47, since the amount of Ti contained in the steel material is too large, the base material was solid-solution strengthened by the solid solution of Ti, and as a result, the HAZ toughness is deteriorated. No. In No. 48, since the amount of Ti contained in the steel material is too small, the amount of inclusions having a circle-equivalent diameter of 0.1 to 2 μm that becomes the nucleus of the intragranular α transformation cannot be secured. Accordingly, the HAZ toughness is deteriorated. No. In No. 49, since the amount of Al contained in the steel material is too large, a large amount of coarse oxide having an equivalent circle diameter exceeding 3 μm is generated, and the HAZ toughness is deteriorated. No. 50 is an example in which the amount of N contained in the steel material is excessive, and the amount of solute N contained in the steel material becomes excessive, and it is considered that the HAZ toughness is deteriorated.

Next, FIG. 3 shows the relationship between the number of oxides with an equivalent circle diameter exceeding 3 μm per 1 mm ² of the observation visual field area and the absorbed energy (vE ₋₄₀ ) at −40 ° C. In FIG. 3, the numbers shown in Tables 5 and 6 below are shown. The results of 1-32 are indicated by ○ and No. The results of 35-40, 49, 53, 54, 61 (examples exceeding 5.0 of the comparative examples) are indicated by ●.

As is apparent from FIG. 3, if the number of oxides with an equivalent circle diameter exceeding 3 μm per observation field area of 1 mm ² is 5.0 or less, it is satisfactory even when heated at 1450 ° C. for 5 seconds. It can be seen that it exhibits excellent HAZ toughness.

Next, No. shown in Table 5 below. 2 and No. 2 shown in Table 6 below. Take 33 and consider. These steel materials are substantially the same except that the number ratio of the inclusion I to the total number of inclusions is different (the number ratio of the inclusion II to the total number of inclusions is less than 40%. ing). When the ratio of the number of inclusions I is less than 30% with respect to the number of all inclusions (No. 33), vE- ₄₀ is less than 130 J, whereas 30% with respect to the number of all inclusions. % (No. 2) shows that vE- ₄₀ is 130 J or more, and the HAZ toughness can be improved. In addition, similar results are shown in No. 1 shown in Table 5 below. 23 and No. shown in Table 6 below. 62 is also obtained. That is, when the number ratio of the inclusion I was 30% or more with respect to the number of all inclusions (No. 23), vE- ₄₀ was 130 J or more, and the HAZ toughness could be improved.

Next, No. shown in Table 5 and Table 6 below. 17 and No. Consider 51. These steel materials are substantially the same except that the number ratio of the inclusions II to the total number of inclusions is different (the number ratio of the inclusions I to the total number of inclusions is less than 30%. ing). When the ratio of the number of inclusions II is less than 40% with respect to the number of all inclusions (No. 51), vE- ₄₀ is less than 130 J, whereas 40% with respect to the number of all inclusions. % (No. 17) shows that vE- ₄₀ is 130 J or more, and the HAZ toughness can be improved. In addition, similar results are shown in No. 1 shown in Table 5 below. 26 and No. shown in Table 6 below. 64 is also obtained. That is, when the number ratio of the inclusions II was 40% or more with respect to the number of all inclusions (No. 26), vE- ₄₀ was 130 J or more, and the HAZ toughness could be improved.

Claims (6)

C: 0.02 to 0.15% (meaning mass%, the same applies to the following components),
Si: 0.5% or less (excluding 0%),
Mn: 2.5% or less (excluding 0%),
P: 0.03% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Al: 0.050% or less (excluding 0%),
N: 0.010% or less (excluding 0%),
Ti: 0.005 to 0.10%,
Zr: 0.0005 to 0.050%,
REM: 0.0003 to 0.015%,
Ca: 0.0003-0.010%, and O: 0.0005-0.010%,
The balance is steel made of iron and inevitable impurities,
(A) When the composition of all oxide inclusions contained in the steel material is measured and converted to a single oxide by mass, the average composition is:
ZrO ₂ : 5 to 50%,
REM oxide (REM is represented by M ₂ O _{3 when} M is represented): 5 to 50%, and CaO: 50% or less (not including 0%), and
(B) Of all the inclusions contained in the steel material,
The number of inclusions with a circle equivalent diameter of 0.1 to 2 μm is 120 or more per 1 mm ^{2 of the} observation visual field area.
The number of oxides with an equivalent circle diameter of more than 3 μm is 5.0 or less per 1 mm ^{2 of the} viewing field area
The number of oxides with an equivalent circle diameter of more than 5 μm is 5.0 or less per 1 mm ^{2 of the} viewing field area,
(C-1) When the composition of all inclusions contained in the steel material is measured, the molar ratio of REM to Zr (REM / Zr) satisfies 0.6 to 1.4 with respect to the number of all inclusions. The ratio of the number of inclusions I containing REM and Zr is 30% or more, and / or (c-2) when the composition of all inclusions contained in the steel material is measured, , REM, Zr, Al, Ca, in which the ratio [(REM + Zr) / (Al + Ca + Ti)] of the total number of moles of REM and Zr to the total number of moles of Al, Ca, and Ti satisfies 0.5 to 1.2. And the steel material excellent in the toughness of the heat affected zone of welding, wherein the number ratio of Ti-containing inclusions II is 40% or more. The steel material is still another element,
The steel material according to claim 1, containing Cu: 2% or less (not including 0%) and / or Ni: 3.5% or less (not including 0%). The steel material is still another element,
The steel material according to claim 1 or 2, containing Cr: 3% or less (not including 0%) and / or Mo: 1% or less (not including 0%). The steel material is still another element,
The steel material according to any one of claims 1 to 3, containing Nb: not more than 0.25% (not including 0%) and / or V: not more than 0.1% (not including 0%). The steel material is still another element,
B: The steel material in any one of Claims 1-4 containing 0.005% or less (0% is not included). A method for producing the steel material according to any one of claims 1 to 5,
When adding REM to the molten steel in which the dissolved oxygen amount Q _Of is adjusted to the range of 0.0003 to 0.01% by mass, the dissolved oxygen amount Q _Of of the molten steel and the added amount Q _{REM of the REM} are expressed by the following formula (1). Adding a satisfactory amount of REM,
When adding REM, Zr, Ti, Ca, and Al to molten steel with the dissolved oxygen amount Q _Of adjusted to the above range, when REM and Zr are a group element and Ti, Ca, and Al are b group elements The method for producing a steel material excellent in toughness of the weld heat affected zone, characterized in that the addition condition of each element satisfies the following (2) and / or (3) below.
2logQ _REM + 3logQ _Of ≤-12.00 (1)
(2) About the said a group element, REM and Zr are added simultaneously, or the other element is added within 5 minutes after adding one element among REM and Zr.
(3) The case where the b group element is added before and / or after the addition of the a group element, and the b group element is added before the addition of the a group element. T1 (minutes) from the start of addition of the first element to the start of addition of the first element among the a group elements, and when the b group element is added after the addition of the a group element, the a group element The time from the start of addition of the last element to the start of addition of the first element among the group b elements is t2 (minutes), and the sum of t1 and t2 is 3 minutes or more. (0 ≦ t1, 0 ≦ t2, where t1 and t2 are not 0.)

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