JPWO2014091604A1 - Steel for welding - Google Patents

Abstract

This welding steel material is mass%, C: 0.05% or more, less than 0.12%, Mn: 1.40% or more, 1.80% or less, S: 0.0020% or more, 0.0080% Hereinafter, Al: 0.020% or more, 0.070% or less, Ti: 0.004% or more, 0.012% or less, B: 0.0005% or more, 0.0020% or less, Mg: 0.0015% Or more, 0.0030% or less, N: 0.0020% or more, 0.0050% or less, O: 0.0007% or more, 0.0020% or less, weld crack sensitivity index Pcm value is 0.16% More than 0.23%, hardenability index DI value is 0.70 or more and 2.30 or less, and Mg · Mn-containing sulfide having a particle diameter of 0.015 μm or more and 0.2 μm or less is 1 mm 2. Including 1.0 × 104 or more and 3.0 × 105 or less per M In · Mn-containing sulfide, the ratio of Mg to the total of Mg and Mn, is 70% to 90% by atomic%.

Description

  The present invention relates to a heat affected zone (hereinafter referred to as “Heat Affected Zone”) in super-high heat input welding such as electroslag welding applied in the assembly of box columns such as high-rise buildings, or electrogas welding applied in shipbuilding and bridges. , Referred to as HAZ). In particular, the HAZ has excellent low temperature toughness of HAZ even when the heat input is 200 kJ / cm or more, for example, about 400 to 500 kJ / cm.

  With the recent rise in building structures, steel columns are becoming larger. In connection with this, the plate | board thickness of the steel materials used for a steel pillar has also increased. When assembling such a large steel column by welding, it is required to weld with high efficiency, and electroslag welding that can weld an extremely thick steel plate in one pass has been widely applied. Yes. In the shipbuilding field and the bridge field, electrogas welding in which a steel plate having a thickness of about 50 mm or more is welded in one pass has been widely applied. When performing these electroslag welding or electrogas welding, a typical heat input range is 200 to 500 kJ / cm, which is so-called super-high heat input welding. Unlike such a large heat input welding (less than 200 kJ / cm heat input) such as submerged arc welding, the super high heat input welding has a heat history of around 1350 ° C. in the vicinity of the weld fusion line (FL) and in the heat history experienced by the HAZ. High temperature residence time becomes extremely long. Therefore, the coarsening of the austenite grains is extremely remarkable, and it is difficult to ensure the low temperature toughness of the HAZ. Therefore, for the purpose of ensuring the safety of welded steel structures such as building structures, ships, bridges, etc. under severe low temperature environments such as −20 ° C., the low temperature toughness improvement of HAZ of such super large heat input welding is achieved. This is a very important issue.

  Conventionally, there are many knowledges and techniques for improving the toughness of HAZ (high heat input welding HAZ) when high heat input welding is performed. However, as described above, the heat history experienced by the HAZ, particularly the residence time at 1350 ° C. or higher, differs greatly between the super-high heat input welding with a heat input of 200 kJ / cm or more and the high heat input welding. Therefore, the conventional high heat input welding HAZ toughness improvement technology cannot be simply applied to the target field of the present invention.

  Conventional techniques for improving the toughness of high heat input welding HAZ are mainly based on two basic techniques. One is an austenite grain coarsening prevention technique using the pinning effect of steel particles, and the other is an effective grain refinement technique using austenite intragranular ferrite transformation.

  For example, in Non-Patent Document 1, as a result of examining the effect of suppressing the growth of austenite grains for various nitrides and carbides in steel, Ti-added steel produces fine TiN particles in the steel, and high heat input welding HAZ. It is disclosed that the austenite grain growth in can be effectively suppressed.

  Patent Document 1 contains 0.04 to 0.10% Al, 0.002 to 0.02% Ti, and 0.003 to 0.05% rare earth element (REM: Rare Earth Metal). A technique for improving the high heat input welding HAZ toughness with a heat input of 150 kJ / cm in steel is disclosed. This is a technology that utilizes the action of REM forming acid / sulfide (composite particles of oxide and sulfide) to prevent coarsening of the HAZ structure during high heat input welding.

Patent Document 2 discloses a Ti oxide having a particle diameter of 0.1 to 3.0 μm and a particle number of 5 × 10 ³ to 1 × 10 ⁷ particles / mm ³ or a composite of Ti oxide and Ti nitride. In steels containing any of the above, it is disclosed that these particles act as ferrite transformation nuclei in a high heat input weld HAZ having a heat input of 100 kJ / cm, thereby reducing the HAZ structure and improving the HAZ toughness. Yes.

  In Patent Document 3, in steel containing an appropriate amount of Ti and S, intragranular ferrite is generated with a composite precipitate of TiN and MnS as a nucleus in a high heat input welded HAZ structure, and the HAZ structure is refined. It is disclosed that HAZ toughness is improved.

  Patent Document 4 contains steel containing 0.005 to 0.08% Al, 0.0003 to 0.0050% B, and further containing 0.03% or less of at least one of Ti, Ca, and REM. Disclosed that BN is formed in the cooling process starting from undissolved REM / Ca acid / sulfide or TiN in high heat input welding HAZ, and ferrite is formed from this to improve high heat input HAZ toughness. Has been.

  Patent Document 5 discloses a composite comprising a Ti-containing oxide and MnS containing 40,000 to 100,000 Mg-containing oxides per square mm and having a particle diameter of 0.20 to 5.0 μm. It is disclosed that super high heat input welding HAZ toughness is improved by suppressing austenite grain growth and promoting intragranular ferrite transformation in steel containing 20 to 400 pieces per square mm.

  In Patent Document 6, in steel containing two or more kinds of MgO, MgS, Mg (O, S) having a particle diameter of 0.005 to 0.5 μm, super high heat input welding is achieved by suppressing austenite grain growth by these fine particles. It is disclosed that HAZ toughness is improved.

  According to Patent Document 7, in steel containing a large amount of (Mg, Mn) S particles having a particle diameter of 0.005 to 0.5 μm, super high heat input welding HAZ toughness is improved by suppressing austenite grain growth by these fine particles. Is disclosed.

However, the above-described technique has the following problems.
The technique disclosed in Non-Patent Document 1 is a technique for suppressing austenite grain growth using nitrides including TiN. Therefore, the effect is exhibited in the large heat input welding, but in the super large heat input welding targeted by the present invention, since the residence time of 1350 ° C. or more is extremely long, most of the TiN is dissolved, and the effect of suppressing the grain growth. Lose. In addition, a part of the coarse micron-sized TiN that remains undissolved may act as a starting point of brittle fracture in a super-high heat input HAZ at −20 ° C., thereby reducing toughness. Therefore, this technique cannot be applied to the toughness of the super high heat input welding HAZ which is the object of the present invention.

  The technique disclosed in Patent Document 1 uses REM acid / sulfide to prevent coarsening of HAZ during high heat input welding. Since the acid / sulfide has higher stability at a high temperature of 1350 ° C. or higher than the nitride, the effect of suppressing grain growth is maintained. However, it is difficult to finely disperse the acid / sulfide. In other words, since the number density of acids and sulfides is low, there is a limit to reducing the austenite grain size of super high heat input weld HAZ even if the pinning effect of individual particles is maintained. You can't improve. In addition, a coarse micron-sized REM acid / sulfide may act as a starting point of brittle fracture in a super-high heat input HAZ at −20 ° C., which may reduce toughness.

  The technique described in Patent Document 2 has a HAZ toughness by refining the HAZ structure by causing any particle of Ti oxide or a composite of Ti oxide and Ti nitride to act as a ferrite transformation nucleus. It is a technology to improve. Considering the high-temperature stability of Ti oxide, the effect is maintained even in super-high heat input welding. However, the crystal orientation of ferrite generated from intragranular transformation nuclei is not completely random, and is affected by the crystal orientation of the parent phase austenite. Therefore, when austenite grains are coarsened by super-high heat input welding, there is a limit to refine the HAZ structure only by intragranular transformation. In addition, a coarse micron-sized Ti oxide or a composite of Ti oxide and Ti nitride may act as a starting point for brittle fracture and reduce toughness in an ultrahigh heat input HAZ at −20 ° C. .

  The technique disclosed in Patent Document 3 is a technique for transforming ferrite from TiN—MnS composite precipitates. This method is effective when the residence time of 1350 ° C. or higher is relatively short as in high heat input welding. However, in ultra-high heat input welding such as electroslag or electrogas welding, the residence time of 1350 ° C. or higher is long, and during this time, a large amount of TiN dissolves, so the ferrite transformation nucleus disappears and the effect is sufficient. It cannot be demonstrated. In addition, coarse micron-sized TiN-MnS composite precipitates may act as a starting point for brittle fracture in a super-high heat input HAZ at −20 ° C., thereby reducing toughness.

  The technique disclosed in Patent Document 4 is a technique for refining the HAZ structure by generating ferrite from REM / Ca acid / sulfide or BN formed on TiN. The effect of the conversion can be expected. However, it is difficult to increase the number of REM / Ca acids / sulfides. Furthermore, since TiN is dissolved, there is a limit to improving the toughness of the super large heat input welding HAZ only by the ferrite transformation. In addition, coarse micron-sized composite precipitates in which BN is precipitated on REM / Ca acid / sulfide or TiN act as a starting point for brittle fracture in -20 ° C ultra-high heat input HAZ and reduce toughness. There is a case.

  The technique disclosed in Patent Document 5 is based on the composite of austenite grain growth suppression by a fine Mg-containing oxide of 0.01 to 0.20 μm and a Ti-containing oxide of 0.20 to 5.0 μm and MnS. This is a technique for improving the super large heat input welding HAZ toughness by promoting intragranular ferrite transformation. However, it is necessary to suppress the amount of Al to 0.005% or less in order to produce Ti-containing oxides, and the advantages of conventional Al-added steel are impaired. That is, in the conventional Al deoxidized steel with an Al content of about 0.010 to 0.5%, the temperature of the molten steel can be easily controlled by utilizing the oxidation heat generated by Al in the steel, and it is inexpensive and stable. Has made it possible to mass-produce new steel. If the Al addition amount is limited to about 0.005% or less as in Patent Document 5, a means for substituting molten steel temperature control by oxidation heat generation of Al, such as heating by a molten steel heating device, is required. Al in molten steel also has a role of preventing molten steel contamination by oxygen in the atmosphere, and it is widely known that Al is effective in securing a material by forming a nitride. Reduction to less than% impairs the advantages of these Al additions.

  The technology disclosed in Patent Document 6 is a steel containing two or more of 0.005 to 0.5 μm MgO, MgS, Mg (O, S), and super high heat input by suppressing austenite grain growth by these fine particles. This is a technique for improving welding HAZ toughness. However, for the production of fine MgO, it is necessary to suppress the amount of Al to 0.01% or less, and it is still a problem to impair the advantages of the above-described addition of Al.

  The technique disclosed in Patent Document 7 is based on the present inventors, and on the premise that 0.015% or more of Al is added, (Mg, Mn) S particles having a particle diameter of 0.005 to 0.5 μm are used. This is a technique for improving the super large heat input welding HAZ toughness by suppressing the austenite grain growth by these fine particles in a steel containing a large amount. However, the evaluation temperature at which the improvement of the HAZ toughness is recognized is −5 ° C., and the HAZ toughness is ensured in a severe low temperature environment such as −20 ° C., in particular, a stable and good value in the Charpy test at −20 ° C. It remained as a challenge.

Japanese Unexamined Patent Publication No. 60-184663 Japanese Unexamined Patent Publication No. 60-245768 Japanese Unexamined Patent Publication No. 2-254118 Japanese Laid-Open Patent Publication No. 61-253344 Japanese Laid-Open Patent Publication No. 9-157787 Japanese Unexamined Patent Publication No. 11-286743 Japanese Unexamined Patent Publication No. 2002-3986

"Iron and Steel", published by Japan Iron and Steel Institute, 61st (1975) No. 11, page 65

  The present invention has been devised in view of the above-described problems. That is, low temperature of HAZ in super large heat input welding with heat input of 200 kJ / cm or more, such as electroslag welding applied in assembling box columns of high-rise buildings, and electrogas welding applied in shipbuilding, bridges, etc. It aims at providing steel materials for welding excellent in toughness on the premise of Al-added steel.

The characteristic of the specific steel material for welding which this invention makes object is as follows.
(A) The required preheating temperature during the y-type weld cracking test is 25 ° C. or less.
(B) Charpy absorbed energy when applying a thermal cycle that simulates the thermal history near the weld fusion line (FL) of the weld heat affected zone (HAZ) of a super high heat input weld joint at a weld heat input of 400 kJ / cm Is more than 100J at -20 ° C.
In consideration of application to the above members, the characteristics of the base material are preferably as follows.
(C) The plate thickness is 40 mm or more and 100 mm or less, particularly 60 mm or more and 80 mm or less, and the tensile strength is 490 MPa or more, particularly 510 MPa or more, in 1/4 part (1/4 t part) of the plate thickness of the base material. 720 MPa or less, yield stress is 355 MPa or more, particularly 390 MPa or more, and Charpy absorbed energy at −40 ° C. is 100 J or more.
In addition, since it becomes difficult to ensure the HAZ toughness when the tensile strength increases, the upper limit of the yield stress may be 650 MPa or 600 MPa, and the upper limit of the tensile strength may be 670 MPa or 650 MPa. You may limit the steel material made into object to a thick steel plate.

In order to solve the above-described problems and achieve the object, the present inventors have disclosed (Mg, Mn) S fine particles having a particle diameter of 0.005 to 0.5 μm disclosed in Patent Document 7. In order to further improve the low-temperature toughness of the Al-added steel that can suppress austenite grain growth, many studies were conducted including the investigation of the types and number of particles effective in suppressing austenite grain growth. As a result, the C content (addition amount) is strictly regulated to 0.05% or more and less than 0.12%, the Si content is strictly regulated to less than 0.10%, and the N content in steel is 0%. .0050% or less, O content in steel is reduced to 0.0020% or less, B content is regulated to 0.0005% or more and 0.0020% or less, and evaluated by hardenability index DI value The steel has a hardenability of 0.70 or more and 2.30 or less, and a particle diameter of 0.015 to 0.2 μm of (Mg, Mn) S, that is, Mg · Mn-containing sulfide is 1 square mm. 1.0 × 10 ^{4 to} 3.0 × 10 ⁵ per unit, and the ratio of Mg in the total of Mg and Mn in (Mg, Mn) S particles is controlled to 70% or more and 90% or less in atomic% Is effective in improving low temperature toughness in HAZ during super large heat input welding. And knowledge to. Based on this new knowledge, the present invention was made based on the knowledge that a steel material for welding excellent in low temperature toughness of HAZ in super large heat input welding can be provided on the basis of Al-added steel.
The “welding steel material” in the present invention corresponds to, for example, JIS G3106 “rolled steel material for welded structure”, JIS G3115 “steel plate for pressure vessel”, and JIS G3126 “carbon steel plate for pressure vessel for low temperature”.

That is, the present invention employs the following.
(1) The steel material for welding according to one embodiment of the present invention is, in mass%, C: 0.05% or more and less than 0.12%, Mn: 1.40% or more, 1.80% or less, S: 0 0020% or more, 0.0080% or less, Al: 0.020% or more, 0.070% or less, Ti: 0.004% or more, 0.012% or less, B: 0.0005% or more, 0.0020 %: Mg: 0.0015% or more, 0.0030% or less, N: 0.0020% or more, 0.0050% or less, O: 0.0007% or more, 0.0020% or less, Si : Less than 0.10%, Ca: 0.0005% or less, REM: 0.0005% or less, P: 0.01% or less, Cu: 1.0% or less, Ni: 1.5% or less, Cr: 0 .6% or less, Mo: 0.4% or less, Nb: 0.02% or less, V: 0.06% or less The balance consists of Fe and inevitable impurities, and the Pcm value, which is a weld crack sensitivity index represented by the following formula 1, is 0.16% or more and 0.23% or less, and the hardenability index represented by the following formula 2. 2. A Mg / Mn-containing sulfide having a DI value of 0.70 or more and 2.30 or less and a particle diameter of 0.015 μm or more and 0.2 μm or less is 1.0 × 10 ⁴ or more per square mm. In the Mg · Mn-containing sulfide containing 0 × 10 ⁵ or less, the ratio of Mg in the total of Mg and Mn is 70% or more and 90% or less in atomic%.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 1
DI = 0.367 × ([C] ^1/2 ) × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3.0 × [Mo]) × (1 + 1.75 × [V]) × (1 + 1.77 × [Al])
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are C, Si, and Mn, respectively. , Cu, Ni, Cr, Mo, V, Al, and the content expressed by mass% of B.

(2) In the steel for welding described in (1) above, the mass may be further limited to Ni: 0.7% or less.

(3) In the steel material for welding described in (1) or (2) above, further, by mass, Cu: 0.5% or less, Cr: 0.3% or less, Mo: 0.10% or less, You may restrict.

(4) In the steel for welding according to any one of (1) to (3), the plate thickness is 40 mm or more and 100 mm or less, the yield stress is 355 MPa or more, the tensile strength is 490 MPa or more and 720 MPa or less, It may be.

  According to the steel for welding shown in the above aspect of the present invention, it is possible to produce a highly reliable welded structure by applying it to a structure to which super-high heat input welding is applied. The effect on is extremely large.

Hereinafter, a steel material for welding according to an embodiment of the present invention will be described.
The steel material for welding according to the present embodiment is premised on the fact that it is a steel material produced by a production method including Al deoxidation, which is an excellent mass production process with a large amount of production results.
The present inventors conducted a detailed investigation and research on the relationship between the structure and toughness of super high heat input welding HAZ. As a result, it was concluded that the super large heat input welding HAZ toughness is limited even if the conventional structure control or toughness improving method of the high heat input welding HAZ is applied as it is. Moreover, it discovered that the austenite grain of super-high heat input welding HAZ needs to be remarkably refined | miniaturized (fine-grained) for toughness improvement.
It is effective to use the pinning effect by the particles in steel to refine the austenite grains. However, even TiN, which is considered to be the most thermally stable among nitrides, is almost dissolved when heated to 1350 ° C. or more for a long time, so that the pinning effect is lost. There are limits to the application of. Therefore, it is essential to use particles that are stable at high temperatures. However, although REM or Ca oxide (including acids and sulfides) of the prior art has relatively high stability at high temperatures, these oxides are sufficient to suppress the austenite grain coarsening of super high heat input welding HAZ. It is extremely difficult to finely disperse in steel.

Conventionally, about 0.2 to 2% of Mn and about 0.002 to 0.02% of S are added to Al deoxidized steel, and it is widely known that MnS is formed. Since this MnS is dissolved at a high temperature, it could not be a particle for refining austenite grains. As a result of a comparative study of various particles on the premise of Al deoxidized steel, the present inventors have found that Mg / Mn-containing sulfide (Mg, Mn) S particles are stable at high temperatures and suitable for fine dispersion. I know that there is. In addition, particles that exert an effect on the suppression of HAZ austenite grain growth are mainly particles of 0.2 μm or less, but fine (Mg, Mn) by controlling Mn, Mg, S, Al content, etc. It has been found that S can be finely dispersed in a large amount in steel.
However, the evaluation temperature at which the effect of improving the HAZ toughness by (Mg, Mn) S particles has been observed so far has been −5 ° C. That is, securing HAZ toughness under a severe low temperature environment such as −20 ° C. has been a problem. When the toughness evaluation temperature is as low as −20 ° C., the effect of improving the toughness due to the refinement of HAZ austenite grains is limited, and the knowledge of the HAZ toughness improving technique disclosed in Patent Document 7 alone is −20. It was difficult to stably obtain the HAZ toughness at ° C.

In response to this problem, the present inventors have made a number of studies for further toughness improvement. As a result, in the (Mg, Mn) S particles, the ratio of Mg in the total of Mg and Mn is controlled, and the C content, Si content, B content, N content, and O content are strictly controlled. It was newly found that the HAZ low temperature toughness can be further improved by strictly regulating the hardenability represented by the DI value.
Details will be described below.

  The present inventors have found that, with respect to the ratio of Mg and Mn in the (Mg, Mn) S particles, as the Mg ratio increases, the particles become more stable at higher temperatures and have a strong austenite grain growth suppressing effect. The (Mg, Mn) S particles identified in Patent Document 7 are sulfides mainly composed of Mn, the ratio of Mg and Mn is 5% by weight, and Mg is 5% or more and 40% or less (in terms of atomic%, Mg was 10.6% or more and 60.1% or less). These particles have a particle composition closer to MnS that is unstable at high temperature than MgS that is stable at high temperature, so the stability of the particles at high temperature is not sufficient, and the HAZ toughness at −20 ° C. is stable and good. I couldn't. However, it is considered that 70% or more of Mn in MnS has been replaced with Mg (Mg, Mn) S particles, that is, the proportion of the total of Mg and Mn in the particles is atomic%, and 70% ≦ Mg ≦ 90 %, 10% ≦ Mn ≦ 30% (Mg, Mn) S particles were found to be extremely stable at high temperatures and easily finely dispersed. The reason why such (Mg, Mn) S particles are stable at high temperatures and easily disperse finely is unknown at present.

  The inventors of Patent Document 7 include the inventors. In the manufacturing process, the super-high heat input high-tensile steel according to Patent Document 7 added Mg before adding a sufficient amount of Al. When the present inventors add Mg before adding a sufficient amount of Al, the proportion of Mg present as a coarse oxide increases, and as a result, Mg in the fine (Mg, Mn) S particles It was newly found that the ratio decreased. That is, the (Mg, Mn) S particles disclosed in Patent Document 7 are sulfides mainly composed of Mn, and the ratio of Mg and Mn is wt%, and Mg is 5% or more and 40% or less (when converted to atomic%, Mg Is 10.6% or more and 60.1% or less). The (Mg, Mn) S particles are not sufficiently stable at high temperatures, and the γ grains in the FL part may be partially coarsened. Even if there are coarse austenite grains in part, the average austenite grain size is fine, so the toughness at −5 ° C. can be satisfied. However, at −20 ° C., coarse ferrite grains, bainite grains, and the like due to some coarse austenite grains become the starting point of fracture, so in the Mn-based (Mg, Mn) S particles disclosed in Patent Document 7, It was difficult to improve the toughness stably.

  The present inventors have conducted many studies for further improving the stability of particles at high temperatures. As a result, 0.020% or more of Al is added prior to the addition of Mg, and it is confirmed that the mixing of Ca and REM can be suppressed to 0.0005% or less, and then Mg is added, so that atomic% is obtained. Thus, it has been found that (Mg, Mn) S particles mainly composed of Mg and having a high atomic ratio of Mg can be obtained stably. And, unlike the (Mg, Mn) S particles identified in Patent Document 7 within the range of chemical components of the welding steel material according to this embodiment manufactured as described above, the stability at a higher temperature is higher. Increased (Mg, Mn) S particles, that is, sulfides with a high atomic ratio of Mg, in which the ratio of Mg and Mn is 70% ≦ Mg ≦ 90% and 10% ≦ Mn ≦ 30% in atomic%, are generated. I understood. Moreover, it discovered that HAZ toughness in -20 degreeC could be improved by using such particle | grains.

Furthermore, when the evaluation temperature of toughness becomes a low temperature such as −20 ° C., a fine embrittlement phase that does not become a problem at −5 ° C. has an adverse effect on toughness, which may inhibit the stabilization of toughness. I found out. The present inventors further reduce the amount of small and small amount of island-like martensite (mixed phase of martensite and austenite, which is a hard brittle structure: MA), in which no adverse effect was observed in the toughness evaluation at −5 ° C. As a result, it was found that the toughness at −20 ° C. was remarkably improved. And in order to reduce island martensite, in addition to strict control of C content, suppression of Si content, strict control of B content and N content, control the index represented by DI value Was found to be effective.
In HAZ when the growth of austenite grains is suppressed by the above-mentioned Mg particles having a high atomic ratio of Mg (Mg, Mn), HAZ has a microstructure mainly composed of fine ferrite and pearlite. In such a structure, island-like martensite was finely dispersed and considered to be less harmful to toughness. However, at −20 ° C., there is an adverse effect on toughness, so the above regulation is necessary. Furthermore, the regulation of the DI value is effective from the point of making the ferrite structure finer.

  Furthermore, if the ferrite structure is not sufficiently fine at −20 ° C., adverse effects of a small amount of island martensite and a small amount of oxides and nitrides described later increase. The inventors of the present invention do not have sufficient suppression of austenite grain growth with (Mg, Mn) S particles to sufficiently refine (fine) ferrite, and it is important to further delay the progress of ferrite transformation. I found out. It is a structure containing finer ferrite, finer pearlite, and finer bainite, and the formation of island martensite is suppressed, so that HAZ toughness at low temperatures is stably improved.

  When the austenite grain growth is suppressed by (Mg, Mn) S particles, the austenite grain interfacial area is large, and therefore the ferrite transformation tends to proceed excessively. Therefore, it is important to optimize the size and fraction of the ferrite by delaying the progress of the ferrite transformation. On the other hand, the present inventors have newly found that the above-described regulation by the DI value is effective as a means for delaying the progress of the ferrite transformation. Furthermore, as a result of repeated studies, in order to obtain a more stable toughness improvement effect by the DI value, etc., the C content, the Si content and the DI value are strictly controlled, and the toughness evaluation at −5 ° C. is adversely affected. It has been newly found that it is effective to reduce the amount of micron-sized oxides and nitrides for which no sapphire was observed. Moreover, in order to control the amount of oxides and nitrides of this micron size, it has been newly found out that it is effective to strictly regulate all upper limits of O content, Ti content and N content. did.

  In the prior art that uses nitrides and oxides such as TiN as austenite grain coarsening suppression and intranuclear transformation ferrite formation nuclei, all upper limits of O content, Ti content, and N content are strictly set. It is difficult to regulate. In the steel material for welding according to the present embodiment, (Mg, Mn) S particles that are sulfides are used for suppressing austenite grain coarsening, so that all upper limits of O content, Ti content, and N content are strictly set. It becomes possible to regulate.

In the present embodiment, the particle diameter and number density (number per unit area) of the (Mg, Mn) S particles are important.
In the present embodiment, the particle diameter of (Mg, Mn) S particles is 0.015 to 0.2 μm. If it is less than 0.015 μm, the austenite grain growth suppressing effect becomes small. A more preferable lower limit of the particle diameter is 0.020 μm. On the other hand, when the number of particles exceeding 0.2 μm increases, the amount of Mg in the steel is limited, resulting in a significant decrease in the number of fine particles, and the austenite grain growth suppressing effect is reduced. The upper limit of the more preferable particle diameter is 0.15 μm, and still more preferably 0.12 μm.
Further, when the number of (Mg, Mn) S particles having a size of 0.015 to 0.2 μm is 1.0 × 10 ⁴ or more per square mm, the effect of suppressing austenite grain growth becomes significant. A more preferable lower limit of the number of particles is 3.0 × 10 ⁴ or more per square mm, and a more preferable lower limit is 4.0 × 10 ⁴ or more per square mm. On the other hand, in order to increase to 3.0 × 10 ⁵ or more, excessive Mg addition is required, which impairs economic efficiency, so the upper limit of the number of (Mg, Mn) S particles is set to 3.0 × 10 ⁵ per square mm. Restricted. A more preferable upper limit value is 2.0 × 10 ⁵ per square mm.

The method for measuring the number of particles is to create an extraction replica from a steel plate (welding steel), and use a transmission electron microscope (TEM) with a characteristic X-ray detector (EDX) to have a size of 0.015 to 0.2 μm. The number of particles is measured per area of at least 1000 μm ² and converted to the number per unit area. For example, when one field of view is observed as 100 mm × 80 mm at a magnification of 20,000 times, the observation area per field is 20 μm ² , so at least 50 fields are observed. If the number of particles of 0.015-0.2 μm at this time is 100 in 50 fields (1000 μm ² ), the number of particles can be converted to 1 × 10 ⁵ per square mm.

Next, it is measured how many (Mg, Mn) S particles exist among the particles whose number has been measured. If the number of particles is large, it will be 1000 or more, so identifying all the particles one by one is a difficult task. For this reason, it is determined whether or not it is (Mg, Mn) S under the following conditions for at least 20 particles or more, and the abundance ratio thereof is determined. To obtain the number of (Mg, Mn) S particles. For example, when the abundance ratio of (Mg, Mn) S is 90% with respect to the number of particles described above, 1 × 10 ⁵ per square mm, the number of (Mg, Mn) S particles is 1 mm 2. It is assumed that there are 9 × 10 ⁴ pieces per unit.

Next, a method for identifying (Mg, Mn) S particles will be described. In this embodiment, the ratio of Mg and Mn to the total of Mg and Mn in the (Mg, Mn) S particles is 70% ≦ Mg ≦ 90% and 10% ≦ Mn ≦ 30% in atomic%. . Any sulfide other than Mg and Mn may be detected in order to exhibit the effect of refining austenite grains as long as it is a sulfide mainly composed of Mg and Mn. In addition, a trace amount of O may be detected in the particles, but if the ratio of S and O is atomic% and 95% ≦ S and the contained O is a trace amount of less than 5% (Mg , Mn) S particles. However, the ratio of S and O is 95% ≦ S in atomic%, and even when the contained O is less than 5%, the particles can be clearly identified as a composite of MnS and MgO. Are not regarded as (Mg, Mn) S particles. The ratio of Mg and Mn and the ratio of S and O are determined by EDX. The electron beam diameter used for this determination is 0.001 to 0.02 μm, the TEM observation magnification is 50,000 to 1,000,000 times, and an arbitrary position in the fine (Mg, Mn) S particles is determined.
When an extraction replica is made from a steel plate, a large number of precipitates other than (Mg, Mn) S particles having a size of 0.015 to 0.2 μm, such as cementite and alloy carbonitride, are generated (Mg, Mn). If it is difficult to measure the number of S particles, hold for about 60 seconds at 1400 ° C. to dissolve particles other than (Mg, Mn) S, and then rapidly cool or heat cycle in which ferrite forms during quenching. A sample with few cementite and alloy carbonitrides may be prepared, and an extraction replica may be prepared therefrom.
Since the (Mg, Mn) S particles are stable at high temperatures, the results do not change even when the above heat cycle is applied.

  In order to disperse the above-mentioned size and number of particles in the steel, in this embodiment, the contents of Mg, Mn, S, and Al are limited as follows as chemical components of the steel material for welding.

Mg: 0.0015% or more and 0.0030% or less Mg is an element essential for the generation of (Mg, Mn) S particles. If the Mg content is less than 0.0015%, a necessary number of (Mg, Mn) S particles cannot be obtained. Moreover, the ratio of Mg in the (Mg, Mn) S particles becomes low. In order to produce a larger amount of fine (Mg, Mn) S particles, addition of 0.0018% or more or 0.0020% or more is more preferable. If the content exceeds 0.0030%, Mg tends to generate an oxide (Mg, Mn), the amount of S is saturated, the HAZ toughness improving effect is saturated, and the economic efficiency is impaired, so the upper limit is 0.0030%. did. For economy, the upper limit may be 0.0027% or 0.025%.

Mn: 1.40% or more and 1.80% or less Mn is an essential element because it is an element constituting (Mg, Mn) S particles. When Mn is contained in an amount of 0.2% or more, fine (Mg, Mn) S particles can be dispersed in a large amount. To sufficiently obtain (Mg, Mn) S particles containing 10% ≦ Mn ≦ 30%. It is necessary to contain 1.40% or more. Further, if it is less than 1.40%, it is disadvantageous for securing strength and HAZ toughness. In order to improve the HAZ toughness, the lower limit of the content may be 1.45% or 1.50%. On the other hand, when Mn exceeds 1.80%, (Mg, Mn) S particles are easily coarsened and the HAZ toughness is lowered, so 1.80% was made the upper limit. In order to improve the HAZ toughness, the upper limit may be 1.75% or 1.70%.

S: 0.0020% or more and 0.0080% or less S is an essential element for generating (Mg, Mn) S particles. If the S content is less than 0.0020%, the amount of (Mg, Mn) S particles is insufficient, so the lower limit was made 0.0020%. In order to produce a larger amount of fine (Mg, Mn) S particles, addition of 0.0025% or more or 0.0030% or more is more preferable. On the other hand, if the content exceeds 0.0080%, the proportion of Mg in the (Mg, Mn) S particles is reduced, and the stability of the particles at high temperatures is insufficient, so fine (Mg, 0.2 μm or less) The number of Mn) S particles is reduced, and the effect of refining γ grains (austenite grains) of super large heat input welding HAZ is reduced. Furthermore, coarse (Mg, Mn) S particles are generated and act as a starting point for brittle fracture. Therefore, the low temperature HAZ toughness decreases. Therefore, the upper limit is set to 0.0080%. A more preferable upper limit of the amount of S is 0.0070%. In order to improve HAZ toughness, the upper limit may be 0.0065%, 0.0060%, or 0.0055%.

Al: 0.020% or more, 0.070% or less Al suppresses the formation of coarse oxides of Mg, and Mg is an essential element for generating fine (Mg, Mn) S particles. . Therefore, a content of 0.020% or more is necessary. In order to produce a larger amount of fine (Mg, Mn) S particles, 0.025% or more or 0.030% or more of Al is more preferable. On the other hand, when the content exceeds 0.070%, a mixed phase of martensite and austenite (MA: Martensite-Austenite Constituent) which is a hard embrittlement structure is easily generated in HAZ, or HAZ embrittlement due to solid solution Al. Causes HAZ toughness to decrease. Therefore, the upper limit was made 0.070%. A more preferable upper limit of the amount of Al is 0.060%. In order to improve HAZ toughness, the upper limit may be 0.055% or 0.050%.

Ca: 0.0005% or less, and REM: 0.0005% or less In the present embodiment, it is necessary to generate fine (Mg, Mn) S particles. For this reason, it is desirable to reduce the content of sulfide-forming elements other than Mg and Mn as much as possible. If the sulfide-forming elements other than Mg and Mn are excessive, a sufficient number of (Mg, Mn) S particles cannot be obtained. Typical elements are Ca and REM, which need to be 0.0005% or less. For this reason, the upper limit of Ca and REM was limited to 0.0005%. A more desirable upper limit is 0.0003%. There is no particular need to limit these lower limits, and these lower limits are 0%.

  The HAZ toughness greatly varies depending on the alloy element content, as well as austenite grain refinement and grain refinement, reduction of coarse cementite and island martensite, and reduction of coarse oxides and nitrides. In addition, it is desirable to contain an appropriate alloy element in order to ensure the strength and toughness of the base material necessary for the structure. Therefore, the content (addition amount) of alloy elements (chemical components) other than the above is limited for the following reason.

C: 0.05% or more and less than 0.12% C is an element that increases the strength of the base material. If less than 0.05%, the effect of improving the strength of the base material is small, so 0.05% was made the lower limit. A more preferable lower limit of the C content is 0.06%. On the other hand, when the C content exceeds 0.12%, cementite and island martensite, which are the starting points of brittle fracture, increase, and the HAZ toughness decreases. In particular, for low temperature toughness at −20 ° C., even a relatively small amount of small cementite or island martensite tends to be the starting point of brittle fracture, and may reduce HAZ toughness, so the upper limit of the C content is strict. Regulation is necessary. A more preferable upper limit value of the C content is 0.10% or 0.09%, and a more preferable upper limit value of the C content is 0.08%.

Si: Less than 0.10% When Si is contained, an island-like martensite phase, which is a hard embrittled structure, easily forms in the HAZ microstructure. Since this island-shaped martensite deteriorates the low temperature toughness of the HAZ, the Si content is less than 0.10%. Although it is desirable that the content is small, reduction of the Si content to less than 0.03% may be accompanied by an increase in cost, and in that case, it is desirable that 0.03% be the lower limit. There is no need to particularly limit the lower limit of the Si amount, and the lower limit is 0%. Although Si is not desirable for improving HAZ toughness, Si may be intentionally added as long as it is less than 0.10%.

Ti: 0.004% or more and 0.012% or less Ti mainly enhances the effect of improving the hardenability by B, and is therefore effective in increasing the strength of the base material and refining the HAZ structure. In order to refine the HAZ structure, it is important to secure the amount of the solid solution B. The solid solution B refines the HAZ structure by delaying the ferrite transformation of the super high heat input HAZ. Since Ti fixes solid solution N as TiN and suppresses the generation of BN, the amount of solid solution B can be ensured. Further, it is effective for refining the base material structure (fine graining) by the effect of suppressing the grain growth of austenite grains by TiN and for refining the HAZ structure heated to 1350 ° C. or less. However, if the content is less than 0.004%, these effects cannot be obtained, so the lower limit is set to 0.004%. In order to reliably exhibit these Ti addition effects, the lower limit may be 0.005% or 0.006%. On the other hand, if the content exceeds 0.012%, coarse TiN is generated and this becomes a starting point of fracture, so that HAZ toughness is lowered. Therefore, the upper limit is set to 0.012%. A more preferable upper limit value of the Ti amount is 0.010% or 0.009%, and a further preferable upper limit value of the Ti amount is 0.008%.

B: 0.0005% or more and 0.0020% or less B is an element that exhibits a significant strength increase effect when controlled cooling and is effective in increasing the base material strength. In addition, since the solute B delays the ferrite transformation in the ultra-high heat input HAZ, it is effective for refining the microstructure. However, if the content is less than 0.0005%, the effect of increasing the strength cannot be obtained, so the lower limit was set to 0.0005%. In order to reliably exhibit these B addition effects, the lower limit may be set to 0.0007% or 0.008%. On the other hand, if the content exceeds 0.0020%, coarse B nitrides and carbon borides are precipitated and serve as starting points for fracture, so that HAZ toughness is lowered. Therefore, the upper limit is set to 0.0020%. A more preferable upper limit of the amount of B is 0.0017%, and a more preferable upper limit of the amount of B is 0.0015% or 0.0013%.

N: 0.0020% or more and 0.0050% or less If the content of N is large, coarse TiN and (Ti, Nb) (C, N) are likely to be generated. These particles serve as starting points for brittle fracture. In the evaluation of the super-high heat input HAZ at −20 ° C., even TiN or (Ti, Nb) (C, N) of several μm becomes a starting point of brittle fracture and causes a reduction in the HAZ toughness, so it is strictly controlled. Further, if the amount of solute N is large, BN is generated and the amount of solute B decreases, which is not preferable. When the amount of solute B is reduced, the effect of solute B delaying the ferrite transformation and refining the HAZ structure and the effect of improving the base material strength are reduced. In particular, in the steel for welding according to the present embodiment, the Ti content is limited to 0.012% or less so as not to generate coarse TiN, so the amount of solute N not fixed to Ti as TiN increases. Cheap. Therefore, it is necessary to strictly limit the N content) from the beginning. Therefore, the upper limit value is set to 0.0050%. A more preferred upper limit is 0.0045% or 0.0040%, and even more preferably 0.0030%. Although it is desirable that the N content is small, the reduction of the N content to less than 0.0020% may involve a cost increase, so 0.0020% was made the lower limit. In order to avoid an increase in cost, 0.0023% or 0.0026% may be set as the lower limit.

O: 0.0007% or more and 0.0020% or less When the O content is large, many coarse oxides are easily generated. A coarse oxide becomes a starting point of fracture, and reduces HAZ toughness. Even when the Al content prior to the addition of Mg is 0.020% or more, the oxygen content exceeds 0.0020% due to air pollution of the molten steel due to special factors such as malfunctions in equipment or operation. Increases the amount of Mg consumed by coarse oxides. As a result, the proportion of Mg in the fine (Mg, Mn) S particles decreases, the number of (Mg, Mn) S particles decreases, and the HAZ toughness may decrease. For this reason, the upper limit of O content was made 0.0020%. A more preferred upper limit is 0.0018% or 0.0016%. Although it is desirable that the O content is small, the reduction of the O content to less than 0.0007% may involve an increase in cost, so 0.0007% was made the lower limit. In order to avoid cost increase, the lower limit may be set to 0.0009% or 0.0011%.

P: 0.010% or less P is an element which causes grain boundary embrittlement and is harmful to toughness. Therefore, it is desirable that the P content is small. If the content exceeds 0.010%, even if HAZ austenite grains are refined by (Mg, Mn) S particles, the HAZ low-temperature toughness decreases, so the content is limited to 0.010%. Preferably, it is 0.009% or less, More preferably, it is 0.008% or less. There is no need to particularly limit the lower limit of the P amount, and the lower limit is 0%.

Cu: 1.0% or less Cu is an element effective for increasing the strength of the base material, and may contain Cu, but if it exceeds 1.0%, the HAZ toughness decreases. Therefore, the Cu content is limited to 1.0% or less. Preferably, it is 0.8% or less, more preferably 0.7% or less, and still more preferably 0.5% or less. Cu may be mixed as an inevitable impurity from scraps or the like during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.

Ni: 1.5% or less Ni has an effect of increasing the strength of the base material by increasing the hardenability and further improves toughness. For this reason, you may contain Ni. However, Ni is an expensive element, and if it exceeds 1.5%, the economy is impaired, so the Ni content is limited to 1.5% or less. Preferably, it is 1.2 or less, more preferably 1.0% or less, and still more preferably 0.7% or less. Ni may be mixed as an inevitable impurity from scrap or the like during the production of molten steel, but there is no need to particularly limit the lower limit, and the lower limit is 0%.

Cr: 0.6% or less Since Cr is effective in increasing the strength of the base material, it may contain Cr. However, if it exceeds 0.6%, island martensite is generated in the HAZ, and the HAZ toughness is lowered. Therefore, the Cr content is limited to 0.6% or less. Preferably, it is 0.4% or less, more preferably 0.3% or less. Although Cr may be mixed as an inevitable impurity from scrap or the like during the production of molten steel, the lower limit is not particularly limited, and the lower limit is 0%.

Mo: 0.40% or less Since Mo is effective in increasing the strength of the base material, it may contain Mo. However, if the content exceeds 0.40%, a hardened structure is formed in the HAZ, and the HAZ toughness is lowered. Therefore, the Mo content is limited to 0.40% or less. Preferably, it is 0.25% or less, more preferably 0.10% or less. Mo may be mixed as an inevitable impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

Nb: 0.020% or less Since Nb is an element effective for increasing the strength of the base material and refining the structure, Nb may be contained. However, if the content exceeds 0.02%, the precipitation of Nb carbonitrides in the HAZ becomes prominent, and the HAZ toughness decreases. Therefore, the Nb content is limited to 0.020% or less. Preferably, it is 0.018% or less, More preferably, it is 0.016% or less. Nb may be mixed as an inevitable impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

V: 0.060% or less Since V is an element effective for increasing the strength of the base material and refining the structure, V may be added. However, if the content exceeds 0.060%, precipitation of carbonitrides in the HAZ becomes remarkable, and the HAZ toughness decreases. Therefore, the V content is limited to 0.060% or less. Preferably, it is 0.050% or less. V may be mixed as an inevitable impurity from scrap or the like during the production of molten steel, but there is no need to particularly limit its lower limit, and the lower limit is 0%.

Moreover, in the steel material for welding which concerns on this embodiment, in order to make the required preheating temperature at the time of a y-type weld crack test into 25 degrees C or less, the Pcm value represented by following formula 1 shall be 0.23% or less. More preferably, it is 0.22% or less or 0.21% or less. On the other hand, if the Pcm value is less than 0.16%, sufficient base material strength or sufficient joint strength may not be obtained. Therefore, the lower limit value of the Pcm value is set to 0.16%. A more preferred lower limit is 0.17%.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 1

Furthermore, in the steel for welding according to the present embodiment, the hardenability of HAZ after super-high heat input welding is increased and the ferrite transformation temperature is lowered to refine the ferrite. The DI value was 0.70 or more. HAZ toughness is improved by refining the ferrite in the ultra-high heat input HAZ. That is, if DI is less than 0.70, even if the austenite grain size is fine, the ferrite transformed from austenite is not sufficiently refined and the toughness is lowered. More preferably, it is 0.75. On the other hand, if the DI value exceeds 2.30, the HAZ hardens and the HAZ toughness decreases, so the upper limit was set to 2.30. The upper limit value of the DI value is more preferably 1.50, and still more preferably 1.30.
DI = 0.367 × ([C] ^1/2 ) × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3.0 × [Mo]) × (1 + 1.75 × [V]) × (1 + 1.77 × [Al])
In the above formulas 1 and 2, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], [B] are It means the content expressed by mass% of C, Si, Mn, Cu, Ni, Cr, Mo, V, Al, and B, respectively.

The steel for welding according to the present embodiment contains or restricts the above components, and the balance contains iron and inevitable impurities. However, in addition to the above components, the weld steel according to the present embodiment contains the following alloy elements for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as an unavoidable impurity from auxiliary materials such as scrap. You may contain.
Since Sb impairs HAZ toughness, the Sb content [Sb] is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. .
Since Sn impairs HAZ toughness, the Sn content [Sn] is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. .
Since As impairs HAZ toughness, the As content [As] is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. .
In order to sufficiently exhibit the above-described effects of the above components, it is preferable to limit Zr, Co, Zn and W) to 0.01% or less or 0.005% or less, respectively.
There is no need to limit the lower limit of Sb, Sn, As, Zr, Co, Zn and W, and the lower limit of each element is 0%. Moreover, even if an alloy element (for example, Si, Ca, REM, P, Ni, Cr, Mo, Nb, V, and Sb) without a lower limit is intentionally added, or due to contamination as an inevitable impurity Even if it exists, if the content is in a claim, the steel material will be interpreted as the claim of this invention.

  The effect of improving the HAZ toughness in the steel for welding according to the present embodiment is effective not only in super high heat input welding but also in high heat input welding (for example, about 100 to less than 200 kJ / cm).

  Next, the manufacturing method of the steel material for welding which concerns on this embodiment is demonstrated.

The steel melting method is, for example, in a state where the molten steel temperature is 1650 ° C. or less, the molten steel O concentration is 0.01% or less, and the molten steel S concentration is 0.02% or less, and Al is added 0.020 prior to the addition of Mg. Add at least%. At that time, after confirming that mixing of Ca and REM could be suppressed to less than 0.0005%, Mg was added, and after adjusting the content of other elements as necessary, by continuous casting By casting, it is possible to obtain a slab containing fine particles of (Mg, Mn) S in which the percentage of Mg in the total of Mg and Mn in the steel is 70% or more and 90% or less in atomic% it can.
Prior to adding Mg, if 0.020% or more of Al is not added and the amount of oxygen in the molten steel is not reduced, then the added Mg will be consumed as a coarse oxide, so fine (Mg, Mn) The amount of Mg that becomes S particles decreases. As a result, the Mg / Mn ratio in the (Mg, Mn) S particles decreases, and the stability of the (Mg, Mn) S particles at high temperatures decreases. For this reason, it is particularly important to add 0.020% or more of Al prior to the addition of Mg.

  Even when Ca and REM are not added intentionally, they may be mixed into the molten steel from refractories used in molten steel pans, flux or slag added for the purpose of desulfurization, alloy raw materials, and the like. Therefore, it is important to suppress mixing of Ca and REM to 0.0005% or less. In order to suppress the mixing of Ca and REM to 0.0005% or less, the amount of Ca and REM contained in the refractory, flux, slag and alloy raw material is controlled. Alternatively, it is managed whether Ca and REM have stable forms and shapes such as oxides and are not easily mixed into molten steel. There is no need to limit the lower limit of Ca and REM, and the lower limit is 0%.

The reason for controlling the order of addition of Al and Mg and the amount of Ca and REM mixed as described above will be described. If Mg is simply added to steel, (Mg, Mn) S particles are hardly generated. The reason is that Mg is a strong deoxidizing element and becomes an oxide. Further, Mg has a high vapor pressure in the molten steel and is an element that is difficult to yield in the molten steel even when added in a large amount. For this reason, it is extremely important to prevent a very small amount of Mg of about 0.0015 to 0.0030% from being consumed as an oxide and to generate (Mg, Mn) S particles. If the Al content when Al is added prior to the addition of Mg is less than 0.020%, the number of (Mg, Mn) S particles cannot be sufficiently obtained. At this time, Mg mainly exists as an oxide as MgAl ₂ O ₄ or MgO. Moreover, since Mg is consumed for the formation of oxides, the proportion of Mg in the total of Mg and Mn in the (Mg, Mn) S particles also decreases.
On the other hand, when the Al content is set to 0.020% or more prior to the addition of Mg, sufficient molten steel deoxidation with Al becomes possible, and the oxygen content in the molten steel can be stably reduced to 0.0020% or less. As a result, the amount of oxide is reduced, and the composition of the oxide is mainly Al ₂ O ₃ and MgO is reduced. Therefore, most of Mg exists as (Mg, Mn) S particles. That is, by adding 0.020% or more of Al prior to the addition of Mg, a large number of fine (Mg, Mn) S particles can be generated.

  Furthermore, in order to generate fine (Mg, Mn) S particles, it is desirable to reduce the content of sulfide-forming elements other than Mg and Mn as much as possible. Typical elements are Ca and REM, and Ca and REM are easier to combine with oxygen and sulfur than Mg and easily form coarse acids and sulfides. Even if Al is added in an amount of 0.020% or more before adding Mg, if Ca or REM exceeds 0.0005% and is mixed in the molten steel, coarse acid / sulfurization containing Ca or REM and Al Many products are formed, and even if Mg is added thereafter, it is difficult to stably obtain fine (Mg, Mn) S particles. In addition, even when Ca or REM is mixed during or after the addition of Mg after adding 0.020% or more of Al, if the mixed amount exceeds 0.0005%, fine (Mg, Mn) S particles It becomes difficult to obtain a stable.

  Heating, rolling, and heat treatment conditions after casting are, for example, controlled rolling / control cooling, direct quenching / tempering after rolling, quenching / tempering after cooling once after rolling, etc. What is necessary is just to select suitably.

  Examples of the present invention are shown below. Steel was melted by a converter and a slab having a thickness of 320 mm was manufactured by continuous casting. Tables 1 and 2 show chemical components of steel types A1 to A52. Steel types A1 to A24 of Table 1 added 0.020% or more of Al prior to the addition of Mg, and Mg was added after confirming that mixing of Ca and REM could be suppressed to 0.0005% or less. . Steel types A27 to A35, A37 to A42, and A45 to A52 in Table 2 add 0.020% or more of Al prior to the addition of Mg, and the mixing of Ca and REM can be suppressed to 0.0005% or less. After confirmation, Mg was added. Steel type A36 in Table 2 added Al prior to the addition of Mg, but the Al content at that time was less than 0.020%. In steel type A43, 0.020% or more of Al was added prior to the addition of Mg, but Mg was added in a state where Ca was excessively mixed. In steel type A44, 0.020% or more of Al was added prior to the addition of Mg, but Mg was added in a state where REM was excessively mixed. For steel types A25 and A26, Al was added after adding Mg.

Tables 3 and 4 show the methods of manufacturing steel sheets (steel materials No. 1 to 52) manufactured using slabs having chemical components of steel types A1 to A52, sheet thicknesses, base material characteristics, and joint toughness evaluation results based on weld reproduction thermal cycles. Indicates. As shown in Tables 3 and 4, steel plates were manufactured by controlled rolling / control cooling method, quenching / tempering method, and direct quenching / tempering method, and the plate thickness was 40-100 mm. Base material strength (yield stress and tensile strength) is the direction parallel to the rolling direction from 1/4 part (1 / 4t part) of the plate thickness of No. 4 round bar tensile test piece specified in JIS Z2241 (L direction) ) And evaluated by the method prescribed in JIS Z 2241. The base material toughness is obtained by collecting an impact test piece specified in JIS Z 2242 in a direction (C direction) perpendicular to the rolling direction from a 1/4 t portion, and by Charpy absorbed energy at −40 ° C. by the method specified in JIS Z 2242. (VE-40) was determined and evaluated. Weldability was evaluated by obtaining a preheating temperature necessary for preventing root cracking by performing coated arc welding with a heat input of 1.7 kJ / mm by the method prescribed in JIS Z 3158. The joint toughness was evaluated by collecting Charpy impact test pieces from the test pieces provided with a heat cycle that reproduced super-high heat input welding with a heat input of 500 kJ / cm. The thermal cycle was held at a peak temperature of 1400 ° C. for 30 seconds, and then cooled to 100 ° C. or less at a cooling rate of 1 ° C./second. The impact test was performed at −20 ° C. (vE-20), and the toughness was evaluated by the average value and the minimum value of 9 repetitions. In addition, the austenite particle size was measured for a sample provided with a thermal cycle that was held at a peak temperature of 1400 ° C. for 100 seconds and then rapidly cooled to 100 ° C. or lower, and further, (Mg, Mn ) The number of S particles was measured according to the method described above. At this time, in the particles whose number is measured, the ratio of Mg to the total of Mg and Mn is 70% or more and 90% or less in atomic%. Tables 3 and 4 show, for reference, values obtained by averaging the ratio (atomic%) of Mg in the sulfide particles containing Mg and Mn having a particle diameter of 0.015 to 0.2 μm for each particle. .
The target values of each characteristic are the base material yield stress of 355 MPa or more, the base material tensile strength of 490 MPa or more and 720 MPa or less, the base material vE-40 of 100 J or more, the required preheating temperature of 25 ° C. or less, and super large heat input welding. The average value of vE-20 to which the reproduced thermal cycle was applied was 150 J or more, and the minimum value was 100 J or more.

As apparent from Tables 3 and 4, the steel material No. 1 to 24 satisfy both the required preheating temperature and the target value of HAZ toughness in a thermal cycle that reproduces super-high heat input welding, and 1 (Mg, Mn) S particles having a particle diameter of 0.015 to 0.2 μm are 1 It is 1.0 × 10 ⁴ or more per square mm, and the austenite grain size is as fine as 150 μm or less. The tensile strength was as high as 490 MPa or more.
On the other hand, the steel material No. 28, 29 and 30, 33, 37, 41, 50 are C content, Si content, P content, Al content, B content, DI value exceeds the upper limit, and austenite grains are fine grains However, the HAZ toughness cannot satisfy the target value for both the average value and the minimum value. Steel No. 32, 39, and 45 have Mn content, Ti content, and N content exceeding the upper limit values, respectively. Since 31, 40, 47, and 49 have insufficient DI values, the minimum value of HAZ toughness, or both the minimum value and the average value cannot satisfy the target value. Steel No. Nos. 34, 36, and 42 have insufficient S content, Al content, and Mg content, the number of (Mg, Mn) S particles is small, the austenite grains are coarse, and the HAZ toughness has both an average value and a minimum value. The target value cannot be satisfied. Steel No. Nos. 35, 43, 44 and 46 have an excessive S content, Ca content, REM content, O content, a small number of (Mg, Mn) S particles, coarse austenite grains, and an average HAZ toughness. Both the value and the minimum value cannot satisfy the target value. Steel No. No. 48 has a Pcm value exceeding the upper limit value and cannot satisfy the target value of 25 ° C. or less for the necessary preheating temperature. Steel No. 25 and 26 are those in which Mg is added and then Al is added, the number of (Mg, Mn) S particles is small, austenite grains are coarse, and the average value of HAZ toughness can satisfy the target value, but the lowest The value cannot meet the target value. Steel No. No. 38 has insufficient Ti content, so that the effect of refining the structure cannot be obtained, and the average value and the minimum value of the HAZ toughness cannot be satisfied. Steel No. 51 is the Cu content, steel No. In No. 52, since the Cr content, the Nb content, and the V content exceed the upper limit, the HAZ toughness cannot satisfy the target value. Steel No. In Nos. 27, 31, 38, 40, and 47, the yield stress and tensile strength of the base material did not satisfy the target values.

  According to the welding steel material of the present invention, it is possible to manufacture a highly reliable welded structure by applying it to a structure to which super-high heat input welding is applied, and its effect on the industry is Very large.

Claims (4)

% By mass
C: 0.05% or more, less than 0.12%,
Mn: 1.40% or more, 1.80% or less,
S: 0.0020% or more, 0.0080% or less,
Al: 0.020% or more, 0.070% or less,
Ti: 0.004% or more, 0.012% or less,
B: 0.0005% or more, 0.0020% or less,
Mg: 0.0015% or more, 0.0030% or less,
N: 0.0020% or more, 0.0050% or less,
O: 0.0007% or more, 0.0020% or less,
Containing
Si: less than 0.10%,
Ca: 0.0005% or less,
REM: 0.0005% or less,
P: 0.01% or less,
Cu: 1.0% or less,
Ni: 1.5% or less,
Cr: 0.6% or less,
Mo: 0.4% or less,
Nb: 0.02% or less,
V: 0.06% or less,
Limited to
The balance consists of Fe and inevitable impurities,
Pcm value which is a weld crack sensitivity index represented by the following formula 1 is 0.16% or more and 0.23% or less,
DI value which is a hardenability index represented by the following formula 2 is 0.70 or more and 2.30 or less,
Containing 1.0 × 10 ⁴ or more and 3.0 × 10 ⁵ or less of Mg · Mn-containing sulfide having a particle size of 0.015 μm or more and 0.2 μm or less per square mm,
In the Mg · Mn-containing sulfide, the proportion of Mg in the total of Mg and Mn is 70% or more and 90% or less in atomic%.
A steel material for welding.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 1
DI = 0.367 × ([C] ^1/2 ) × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36 × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3.0 × [Mo]) × (1 + 1.75 × [V]) × (1 + 1.77 × [Al])
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are C, Si, and Mn, respectively. , Cu, Ni, Cr, Mo, V, Al, and the content expressed by mass% of B. Furthermore, in mass%,
Ni: 0.7% or less,
The steel material for welding according to claim 1, wherein Furthermore, in mass%,
Cu: 0.5% or less,
Cr: 0.3% or less,
Mo: 0.10% or less,
The steel material for welding according to claim 1 or 2, characterized by being limited to: The plate thickness is 40 mm or more and 100 mm or less,
Yield stress is 355 MPa or more,
Tensile strength is 490 MPa or more and 720 MPa or less,
The steel for welding according to claim 1 or 2, wherein