JP2016008341A - High-tensile strength steel for welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide, on the assumption of Al-added steel, a high-tensile strength steel for welding that is further more excellent in low temperature ultra-high heat input welding HAZ toughness than conventional one.SOLUTION: There is provided a high-tensile strength steel for welding in which, in mass%, C, Mn, S, Al, Ti, B, Mg, Zr, N, and O are individually contained by a specified range, Si, P, Ca, REM, Cu, Ni, Cr, Mo, Nb, and V are limited in a specified range, the balance is composed of Fe and inevitable impurities. A Pcm value of the formula 1 is 0.16 to 0.23%, a hardenability index DI value of the formula 2 is 1.00 to 3.00, and (Mg,Zr)S particles each having a particle size of 0.015 to 0.2 μm are contained by 1.0×10to 3.0×10pieces/mm. 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+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])... formula 2.

Description

The present invention relates to a high-strength steel for welding excellent in low-temperature toughness of a welding heat-affected zone in super-high heat input welding such as electrogas welding.
There is provided a welding technique of super-high heat input welding such as electroslag welding applied in the assembly of box columns in high-rise buildings or the like, or electrogas welding applied in shipbuilding and bridges. In the heat affected zone (Heat Affected Zone: hereinafter referred to as HAZ) in this super large heat input welding, a steel material for welding excellent in low temperature toughness is required. In particular, the present invention relates to a technology having excellent low-temperature toughness of HAZ even when the heat input is 200 kJ / cm or more, for example, super-high heat input welding of about 400 to 1000 kJ / cm.

  With the recent increase in the height of building structures, steel pillars have become larger, and the thickness of steel materials used for steel pillars has also increased. Furthermore, when assembling such a large steel column by welding, it is required to perform welding with high efficiency, so that electroslag welding that can weld an extremely thick steel plate in one pass is widely applied. It has become to. 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 1000 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.
For this reason, in HAZ, austenite grains are extremely coarse, and it is difficult to ensure low temperature toughness. Therefore, achieving the low temperature toughness improvement of the HAZ for super high heat input welding is an extremely important issue for ensuring the safety of welded steel structures such as building structures, ships and bridges in severe low temperature environments. It has become.

  In order to improve toughness, it is necessary to prevent coarsening of the crystal grain size, and some of the inventors have included MgO, MgS, Mg (O, S) and (Mg, Mn) S particles. The technique which suppresses austenite grain growth using such fine particles and improves super large heat input welding HAZ toughness is proposed (for example, refer to Patent Documents 1 to 3).

In the techniques described in Patent Documents 1 to 3, improvement in HAZ toughness is recognized at evaluation temperatures up to −20 ° C. However, it remains a problem to secure HAZ toughness under a severe low temperature environment such as −30 ° C., in particular, to obtain a stable and stable value in a Charpy test at −30 ° C.
By the way, Zr is known as an element that forms oxides and nitrides in steel materials, and improves HAZ toughness by using Mg oxides and sulfides and Zr oxides and nitrides. A method has been proposed (see, for example, Patent Documents 4 and 5).
Although the techniques described in Patent Documents 4 and 5 are effective in improving the high heat input welding HAZ toughness, the effect on the super large heat input welding HAZ toughness is unknown.

JP-A-11-286743 Japanese Patent Laid-Open No. 2002-3986 JP2013-204118A JP 2000-54065 A JP 2003-49236 A

  An object of the present invention is to provide a high-strength steel for welding excellent in super high heat input welding HAZ toughness at a lower temperature than before, on the premise of an Al-added steel.

Specific characteristics of the high-strength steel for welding targeted by the present invention are as follows.
(1) The high-strength steel for welding according to an aspect of the present invention is mass%, C: 0.05% or more and less than 0.10%, 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, Zr: 0.0010% or more, 0.0050% or less, N: 0.0020% or more, 0.0050% or less, O: 0 0007% or more and 0.0020% or less, Si: less than 0.10%, P: 0.01% or less, Ca: 0.0005% or less, REM: 0.0005% or less, Cu: 1. 0% or less, Ni: 1.5% or less, Cr: 0.6% or less, Mo: 0.40% or less, b: 0.020% or less, V: limited to 0.060% or less, the balance being Fe and unavoidable 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 1.00 or more, (Mg, Zr) S having a particle size of not more than 3.00 and a particle size of not less than 0.015 μm and not more than 0.2 μm, including 1.0 × 10 ⁴ or more and 3.0 × 10 ⁵ or less per square mm, In the (Mg, Zr) S, the proportion of Mg in the total of Mg and Zr 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 high-strength steel for welding described in (1) above, further, in mass%, Ni: 0.7% or less, Cu: 0.5% or less, Cr: 0.3% or less, Mo: 0 It may be limited to 10% or less.
(3) In the high-tensile steel for welding described in (1) or (2) above, the plate thickness is 40 mm or more and 100 mm or less, the yield stress is 355 MPa or more, and the tensile strength is 490 MPa or more and 720 MPa or less. There may be.

According to the high-strength steel for welding according to the present invention, it is possible to manufacture a highly reliable structure to which super-high heat input welding is applied, and the effect on the industry is extremely great.
Moreover, the high-tensile steel of the present invention premised on Al-added steel can easily control the molten steel temperature by utilizing oxidation heat generated by Al in the steel making process. Further, Al in the molten steel also has a role of preventing molten steel contamination by oxygen in the atmosphere and forms Al nitride, which is effective for securing the material of the high-strength steel according to the present invention.
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”.

Hereinafter, a high strength steel for welding according to an embodiment of the present invention will be described.
The high-strength steel 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.
In Al deoxidation, the temperature of molten steel can be easily controlled by utilizing oxidation heat generated by Al, and Al in molten steel also has a role of preventing molten steel contamination by oxygen in the atmosphere. On the other hand, if the Al addition amount is limited to about 0.005% or less, a means for substituting the molten steel temperature control by the oxidation heat generation of Al, such as heating by a molten steel heating device, is required.

The present inventors conducted a detailed investigation and research on the relationship between the structure and toughness of super large heat input welding HAZ (welding heat affected zone). 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 comparative study of various particles on the premise of Al deoxidized steel, the present inventors found that Mg / Mn-containing sulfide (Mg, Mn) S particles are stable at high temperatures and suitable for fine dispersion I know that. Also, by controlling the contents of Mn, Mg, S, Al, etc., a large amount of fine (Mg, Mn) S of 0.2 μm or less is effective in suppressing the growth of austenite grains in HAZ. We know that it can be dispersed.
However, the evaluation temperature at which the effect of improving the HAZ toughness by the (Mg, Mn) S particles has been observed so far is −20 ° C. When the temperature becomes as low as −30 ° C., the effect of improving the toughness by refining HAZ austenite grains is It was limited. Therefore, the present inventors have further investigated the problem of obtaining HAZ toughness stably under a severe low temperature environment such as −30 ° C.

  As a result, in the (Mg, Zr) S particles different from the (Mg, Mn) S particles, controlling the ratio of Mg in the total of Mg and Zr, and further, the C content, the Si content, and the B content It was newly found that the HAZ low temperature toughness can be further improved by strictly regulating the hardenability represented by the DI value after strictly regulating the N content and the O content.

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. (Mg, Mn) S particles are Mg-based sulfides, and the proportion of Mg and Mn is atomic% and Mg is 70% or more and 90% or less. The (Mg, Mn) S particles have a particle composition close to that of MgS that is stable at high temperatures, compared to MnS particles that are unstable at high temperatures, and thus the particles have high stability at high temperatures. In some cases, coarse austenite grains were formed.

  If the toughness at −20 ° C. in high-tensile steel, even if there are some coarse austenite grains, the average austenite grain size is fine, so it can be satisfied. However, at −30 ° C., since coarse ferrite grains, bainite grains and the like due to some coarse austenite grains become the starting points of fracture, it is difficult to improve stable toughness with (Mg, Mn) S grains. . Therefore, the present inventors have made many studies in order to further improve the high temperature stability of (Mg, Mn) S particles.

As a result, in order to further improve the high-temperature stability of the Mg-based sulfide and to suppress the decrease in the pinning effect due to the decrease in the volume fraction of the particles, elements other than Mn, that is, Zr as the third element It has been found that the addition of is effective. Furthermore, if the proportion of the Mg and Zr in the particles is (%, 70% ≦ Mg ≦ 90%, 10% ≦ Zr ≦ 30%), the particles are extremely stable at high temperatures. In addition, the present inventors have found that fine dispersion can be easily performed and a sufficient volume fraction of particles can be obtained.
However, the reason why such (Mg, Zr) S particles are stable at high temperatures and easily disperse finely is unknown at present.

  In HAZ when the growth of austenite grains is suppressed by (Mg, Zr) S grains containing Mg · Zr, the microstructure is mainly composed of fine ferrite and pearlite. In such a microstructure, island-like martensite (mixed phase of martensite and austenite, which is a hard embrittled structure: MA) is finely dispersed and has a low degree of harmfulness to toughness. did not become. However, it has been found that when the evaluation temperature of toughness becomes a low temperature such as −30 ° C., the fine embrittlement phase has an adverse effect on toughness and may inhibit toughness stabilization.

Further, when austenite grain growth is suppressed by (Mg, Zr) S particles, the ferrite transformation tends to proceed excessively because the austenite grain interface area is large. Therefore, it is important to optimize the size and fraction of the ferrite by delaying the progress of the ferrite transformation. Furthermore, it was found that in the low temperature range where the evaluation temperature of toughness is −30 ° C., in addition to a small amount of island martensite, the adverse effect on the toughness of oxides and nitrides also increases.
On the other hand, in order to reduce island martensite, the present inventors, in addition to strict control of the C content and Si content, the index represented by the DI value is finely divided by (Mg, Zr) S particles. It was found that it is effective to control the optimum value according to the austenite grain size. Further, it has been found that regulation by DI value and addition of B are effective as means for delaying the progress of ferrite transformation.

In this way, the metal structure of HAZ is controlled by the regulation of the amount of C, Si and DI values, the addition of B, etc., the formation of island martensite is suppressed, fine ferrite, fine pearlite and fine particles Thus, the HAZ toughness at a low temperature can be stably improved. In particular, the HAZ toughness can be stably improved even in the low temperature range of −30 ° C. as well as in the low temperature range of −20 ° C.
Furthermore, it has been found that reducing the amounts of micron-sized oxides and nitrides is effective for obtaining the effect of improving toughness more stably. Further, it has been found that it is effective to strictly control all upper limit values of the O content, the Ti content and the N content in order to control the amounts of the micron-sized oxide and nitride.

  In the prior art that uses nitrides and oxides such as TiN as the nucleation of austenite grain coarsening and intragranular ferrite, the upper limits of all O, Ti, and N contents are strictly regulated. Difficult to do. In the high-strength steel for welding according to the present embodiment, (Mg, Zr) S particles which are sulfides are used for suppressing austenite grain coarsening, so all upper limits of O content, Ti content, and N content. Can be strictly regulated.

In this embodiment, the particle diameter and number density (number per unit area) of (Mg, Zr) S particles are important.
In the present embodiment, the particle diameter of (Mg, Zr) S particles is set to 0.015 μm or more and 0.2 μm or less. 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.

In this embodiment, when the number of (Mg, Zr) S particles having a size of 0.015 μm or more and 0.2 μm or less is 1.0 × 10 ⁴ or more per square mm, the austenite grain growth suppressing effect is obtained. Become prominent. A more preferable lower limit of the number of particles is 3.0 × 10 ⁴ or more per square mm, and a further preferable lower limit is 4.0 × 10 ⁴ or more per square mm. On the other hand, in order to increase the number of (Mg, Zr) S particles to 3.0 × 10 ⁵ or more, excessive Mg addition is required, which impairs the economy, so the upper limit of the number of (Mg, Zr) S particles is 1 square. The number was limited to 3.0 × 10 ⁵ per mm. 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, Zr) 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 identified whether or not it is (Mg, Zr) S for at least 20 particles under the following conditions, its abundance ratio is determined, and the abundance ratio of (Mg, Zr) S in the previously determined number of particles. To obtain the number of (Mg, Zr) S particles. For example, when the existence ratio of (Mg, Zr) S is 90% with respect to the number of particles described above, 1 × 10 ⁵ per square mm, the number of (Mg, Zr) S particles is 1 mm 2. It is assumed that there are 9 × 10 ⁴ per unit.

Next, a method for identifying (Mg, Zr) S particles will be described. In the present embodiment, the ratio of Mg and Zr to the total of Mg and Zr in the (Mg, Zr) S particles is 70% ≦ Mg ≦ 90% and 10% ≦ Zr ≦ 30% in atomic%. . Since sulfides mainly composed of Mg and Zr exhibit an austenite grain refinement effect, elements other than Mg and Zr may be detected.
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 , Zr) S particles. However, even if the ratio of S and O is 95% ≦ S in atomic%, and the contained O is less than 5%, the particles are clearly ZrS and MgO composites or MgS and ZrO composites. If it can be identified that it is, it is not regarded as (Mg, Zr) S particles.

The ratio of Mg and Zr and the ratio of S and O are determined by EDX (energy dispersive X-ray analysis). The electron beam diameter used for this determination is 0.001 to 0.02 μm, the TEM (transmission electron microscope) observation magnification is 50,000 to 1,000,000 times, and an arbitrary position in the fine (Mg, Zr) S particles is Quantify. When (Mg, Zr) S particles contain Mn, there is an effect on the stability of the particles at a high temperature. Therefore, the sample rapidly cooled after holding at 1400 ° C. for 120 seconds, that is, sufficiently stable at 1400 ° C. An extraction replica is made using a sample in which only unstable particles are left and unstable particles are dissolved, and the ratio of Mn in the particles, that is, the ratio of Mn / (Mg + Zr + Mn) in atomic% is measured. .
In this measurement method, the ratio of Mn / (Mg + Zr + Mn) in particles in the atomic% is less than 10%, and the ratio of Mg and Zr is 70% ≦ Mg ≦ 90% and 10% in atomic percent. If ≦ Zr ≦ 30%, the high-temperature stability of the particles is sufficient, and can be considered as the (Mg, Zr) S particles of the present invention.

When an extraction replica is made from a steel plate, a large number of precipitates other than (Mg, Zr) S particles having a size of 0.015 to 0.2 μm, such as cementite and alloy carbonitride, are generated (Mg, Zr). 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, Zr) S, and then perform a thermal cycle in which ferrite is generated during rapid cooling or during rapid cooling. A sample with few cementite and alloy carbonitrides may be prepared, and an extraction replica may be prepared therefrom.
Since the (Mg, Zr) 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, Zr, S, and Al are limited as follows as chemical components of the steel for welding.
Mg: 0.0015% or more and 0.0030% or less Mg is an essential element for generating (Mg, Zr) S particles. If the Mg content is less than 0.0015%, a necessary number of (Mg, Zr) S particles cannot be obtained. Moreover, the ratio of Mg in the (Mg, Zr) S particles is lowered. In order to produce a larger amount of fine (Mg, Zr) S particles, 0.0018% or more or 0.0020% or more of Mg is more preferable. If the Mg content exceeds 0.0030%, Mg tends to form an oxide (Mg, Zr), the amount of S is saturated, the HAZ toughness improving effect is saturated, and the economic efficiency is impaired, so the upper limit. Was 0.0030%. For economy, the upper limit of the Mg content may be 0.0027% or 0.0025%.

Zr: 0.0010% or more and 0.0050% or less Zr is an essential element for generating (Mg, Zr) S particles. When the Zr content is less than 0.0010%, a necessary number of (Mg, Zr) S particles cannot be obtained. In order to produce a larger amount of fine (Mg, Zr) S particles, it is more preferable to add Zr in an amount of 0.0015% or more or 0.0020% or more. If Zr is contained in an amount of more than 0.0050%, Zr tends to generate an oxide (Mg, Zr), the amount of S is saturated, the effect of improving HAZ toughness is saturated, and economic efficiency is impaired, so the upper limit of Zr content The value was 0.0050%. For economy, the upper limit of the Zr content may be 0.0040% or 0.0030%.

Mn: 1.40% or more, 1.80% or less Mn needs to be added 1.40% or more in order to secure strength and HAZ toughness. In order to improve the HAZ toughness, the lower limit of the Mn content may be 1.45% or 1.50%. On the other hand, if Mn exceeds 1.80%, the HAZ toughness is lowered, so 1.80% was made the upper limit. In order to improve the HAZ toughness, the upper limit of the Mn content 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, Zr) S particles. If the S content is less than 0.0020%, the amount of (Mg, Zr) S particles is insufficient, so the lower limit of the S content is set to 0.0020%. In order to produce a larger amount of fine (Mg, Zr) S particles, it is more preferable to add S in an amount of 0.0025% or more or 0.0030% or more. On the other hand, when the content of S exceeds 0.0080%, the ratio of Mg in the (Mg, Zr) S particles becomes low, and the stability of the particles at high temperatures becomes insufficient. The number of Mg, Zr) S particles decreases, and the γ grain (austenite grain) refinement effect of the super high heat input welding HAZ is reduced. Furthermore, coarse (Mg, Zr) S particles are generated and act as a starting point for brittle fracture. Therefore, the low temperature HAZ toughness decreases. Therefore, the upper limit value of the S content 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 of the S content may be 0.0065%, 0.0060%, or 0.0055%.

Al: 0.020% or more and 0.070% or less Al is an essential element for suppressing the generation of coarse oxides of Mg and generating fine (Mg, Zr) S particles. . Therefore, a content of 0.020% or more is necessary. In order to produce a larger amount of fine (Mg, Zr) S particles, 0.025% or more or 0.030% or more of Al is more preferable. On the other hand, when Al is contained in an amount exceeding 0.070%, island martensite, which is a hard embrittled structure, is easily generated in HAZ, and HAZ embrittlement due to solute Al occurs, so that HAZ toughness decreases. Therefore, the upper limit of the Al content is set to 0.070%. A more preferable upper limit of the amount of Al is 0.060%. In order to improve HAZ toughness, the upper limit of Al content 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, Zr) S particles. For this reason, it is desirable to reduce the content of sulfide-forming elements other than Mg and Zr as much as possible. When the sulfide-forming elements other than Mg and Zr are excessive, a sufficient number of (Mg, Zr) S particles cannot be obtained. Typical elements are Ca and REM (rare earth metal elements), 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 need to particularly limit the lower limits of Ca and REM, 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 0.10% C is an element that increases the strength of the base material. If the C content is 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 is 0.10% or more, cementite and island martensite, which are the starting points of brittle fracture, increase and the HAZ toughness decreases, so the upper limit of the C content is less than 0.10%. . In particular, for low temperature toughness at −30 ° C., a relatively small amount of small cementite or island martensite is likely to be the starting point of brittle fracture, and may reduce HAZ toughness. Regulation is necessary. A more preferable upper limit value of the C content is 0.09% or 0.08%, and a more preferable upper limit value of the C content is 0.07%.
Si: Less than 0.10% When Si is added, 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 Si content is small, the reduction of the Si content to less than 0.03% may be accompanied by an increase in cost. In this case, it is desirable that the Si content be 0.03%. There is no need to particularly limit the lower limit of the Si amount, and the lower limit is 0%.

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 Ti content is less than 0.004%, these effects cannot be obtained, so the lower limit was set to 0.004%. In order to reliably exhibit these Ti addition effects, the lower limit of the Ti content may be 0.005% or 0.006%. On the other hand, if Ti is contained in excess of 0.012%, coarse TiN is generated and this becomes the starting point of fracture, so the HAZ toughness decreases. Therefore, the upper limit of the Ti content is set to 0.012%. A more preferable upper limit value of the Ti content is 0.010% or 0.009%, and a further preferable upper limit value of the Ti content 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, when the B content is less than 0.0005%, the effect of increasing the strength cannot be obtained, so the lower limit is set to 0.0005%. In order to reliably exhibit these B addition effects, the lower limit of the B content may be set to 0.0007% or 0.0008%. On the other hand, if B is contained in excess of 0.0020%, coarse B nitrides or carbon borides are precipitated and serve as starting points for fracture, so that HAZ toughness is lowered. Therefore, the upper limit of the B content is set to 0.0020%. A more preferable upper limit value of the B content is 0.0017%, and a more preferable upper limit value of the B content 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 −30 ° C., even TiN or (Ti, Nb) (C, N) of several μm becomes a starting point of brittle fracture and causes a reduction in HAZ toughness, so that it is strictly controlled. Further, if the amount of solute N is large, BN is generated and the amount of solute B is reduced, 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. For this reason, the upper limit of N content was made into 0.0050%. The upper limit value of the N content is more preferably 0.0045% or 0.0040%, and still 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, the N content may be 0.0023% or 0.0026% 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 O content exceeds 0.0020% due to air pollution of the molten steel due to special factors such as equipment or operational problems. In some cases, the amount of Mg consumed by the coarse oxide increases. As a result, the proportion of Mg in the fine (Mg, Zr) S particles decreases, the number of (Mg, Zr) S particles decreases, and the HAZ toughness may decrease. For this reason, the upper limit of O content was made 0.0020%. A more preferable upper limit of the O content 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 an increase in cost, the lower limit of the O content may be 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 P content exceeds 0.010%, even if the HAZ austenite grains are refined by (Mg, Zr) S particles, the HAZ low-temperature toughness decreases, so the content is limited to 0.010% or less. The P content is preferably 0.009% or less, and more preferably 0.008% or less. There is no need to particularly limit the lower limit of the P amount, and the lower limit is 0%.

The high-tensile steel for welding according to this embodiment may further contain one or more of Cu, Ni, Cr, Mo, Nb, and V.
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. The Cu content is preferably 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 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%.

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 the content exceeds 1.5%, the economy is impaired, so the Ni content is limited to 1.5% or less. The Ni content is preferably 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, when adding Cr, it limits to 0.6% or less. About Cr content, Preferably it is 0.4% or less, More preferably, it is 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 Mo is contained in excess of 0.40%, a hardened structure is generated in HAZ, and HAZ toughness is reduced. Therefore, when it contains Mo, it limited to 0.40% or less. The Mo content is preferably 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, when Nb exceeds 0.020%, precipitation of Nb carbonitride in HAZ becomes remarkable, and HAZ toughness decreases. Therefore, the Nb content is limited to 0.020% or less. The Nb content is preferably 0.018% or less, more preferably 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 contained. However, if the V 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, the V content 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%.

Further, in the steel for welding according to the present embodiment, in order to set the necessary preheating temperature at the time of the y-type weld crack test (JIS Z 3158) to 25 ° C. or less, the Pcm value represented by the following formula 1 is set to 0.23. % Or less. The Pcm value is more preferably 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 preferable lower limit of the Pcm value is 0.17%.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] Formula 1
In the above formula 1, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, It means the content expressed by mass% of Cu, Ni, Cr, Mo, V, B.

Furthermore, in the high-tensile steel for welding according to the present embodiment, the ferrite is refined by increasing the hardenability of HAZ after super-high heat input welding and lowering the ferrite transformation temperature. The hardenability index DI value was 1.00 or more.
HAZ toughness is improved by refining the ferrite in the ultra-high heat input HAZ. That is, if the DI value is less than 1.00, even if the austenite grain size is fine, the ferrite transformed from austenite is not sufficiently refined and the toughness is lowered. The DI value is more preferably 1.10. On the other hand, if the DI value exceeds 3.00, the HAZ hardens and the HAZ toughness decreases, so the upper limit was set to 3.00. The upper limit value of the DI value is more preferably 2.70, and further preferably 2.50.
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 formula 2, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [Al] are C, Si, Mn, It means the content expressed by mass% of Cu, Ni, Cr, Mo, V, and Al.

  The high-strength steel for welding according to this embodiment contains or restricts the above components, and the balance contains iron and inevitable impurities. However, in the high-strength steel for welding according to the present embodiment, in addition to the above components, for the purpose of further improving the strength, toughness and the like of the steel material itself, or as an unavoidable impurity from auxiliary materials such as scrap, the following An alloy element may be contained.

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 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, 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 high-tensile steel (steel material) will be interpreted as the claim of this invention.
The effect of improving the HAZ toughness in the high-strength 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 high strength steel 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, the ratio of Mg to the total of Mg and Zr in the steel is 70% or more and 90% or less in atomic% to obtain a slab containing fine particles of (Mg, Zr) S. Can do.

  The heating, rolling, and heat treatment conditions after casting the high-strength steel for welding according to the present embodiment are, for example, controlled rolling / control cooling, direct quenching / quenching after rolling, depending on the target mechanical properties of the base steel material. What is necessary is just to select suitably, such as quenching and tempering after cooling and once cooling after rolling.

If it is the high-tensile steel for welding according to the present embodiment obtained as described above, electroslag welding applied in the assembly of box columns such as high-rise buildings, or electrogas applied in shipbuilding, bridges, etc. Even if it is used for welding of super-high heat input welding such as welding, it is possible to provide performance with excellent low temperature toughness of the weld heat affected zone (HAZ).
In particular, the above-described high-tensile steel for welding has a heat input of 200 kJ / cm or more, and has excellent HAZ low-temperature toughness even in super-high heat input welding of, for example, about 400 to 1000 kJ / cm. Further, the high-strength steel for welding can provide high-tensile steel excellent in low-temperature toughness at -30 ° C as well as excellent in low-temperature toughness at -20 ° C.
More specifically, as an example, a high-strength steel having a plate thickness of 40 mm or more and 100 mm or less and having excellent low-temperature toughness with a yield stress of 355 MPa or more, a tensile strength of 490 MPa or more and 720 MPa or less can be provided.

Examples of the present invention will be described below. However, the following examples are examples of the present invention, and the present invention is not limited to the examples described 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 A50.

  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 A25 to A33, A35 to A41, and A44 to A50 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 A34 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 A42, 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 A43, 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.

  Tables 3 and 4 show the methods of manufacturing steel sheets (steel materials No. 1 to 50) manufactured using slabs having chemical components of steel types A1 to A50, sheet thicknesses, base material characteristics, and joint toughness evaluation results by welding reproduction thermal cycle. 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 metal 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 −50 ° C. by the method specified in JIS Z 2242. (VE-50) 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 heat cycle was held at a peak temperature of 1400 ° C. for 60 seconds, and then cooled to 100 ° C. or less at a cooling rate of 1 ° C./second.
The impact test was performed at −30 ° C. (vE-30), 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 rapidly cooled to 100 ° C. or less after being held at a peak temperature of 1400 ° C. for 120 seconds, and (Mg, Zr) having a particle size of 0.015 to 0.2 μm. ) 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 in the total of Mg and Zr 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 sulfide particles containing Mg and Zr 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-50 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-30 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, Zr) S particles having a particle diameter of 0.015 to 0.2 μm are 1 It contains 1.0 × 10 ⁴ or more per square mm and 2.95 × 10 ⁵ or less, and the austenite grain size is 99 μm or less. The tensile strength was also high at 490 MPa or more, specifically 539 to 718 MPa.

On the other hand, steel material No. Since No. 25 has insufficient C content, the base material strength is insufficient.
Steel No. Since the Ti content of 36 is insufficient, the effect of improving the hardenability of B is insufficient, the strength of the base material is insufficient, the effect of refining the structure cannot be obtained, and the target value of the minimum value of HAZ toughness cannot be satisfied.
Steel No. 38 and 46 have insufficient B content and DI value, and Pcm value and DI value, respectively, and even if the austenite grains are fine grains, the HAZ toughness cannot satisfy the target value for both the average value and the minimum value. The material strength is not satisfactory.
Steel No. 26, 28, 30, 31, 35, 39, 44, and 48 are C content, Si content, Mn content, P content, Al content, B content, N content, and DI value are upper limit values, respectively. Even if the austenite grains are fine grains, the HAZ toughness cannot satisfy the target value for both the average value and the minimum value.

Steel No. In Nos. 27 and 37, the Si content and the Ti content exceed the upper limit, respectively, and even if the austenite grains are fine, the minimum value of the HAZ toughness cannot satisfy the target value.
Steel No. No. 29 has insufficient Mn content and DI value, and even if the austenite grains are fine, the HAZ toughness cannot satisfy the target value for both the average value and the minimum value.
Steel No. 32, 34, 40, 41 have insufficient S content, Al content, Mg content, Zr content, (Mg, Zr) the number of S particles is small, austenite grains are coarse, and HAZ toughness is Neither average nor minimum can satisfy the target value.
Steel No. 33, 42, 43, and 45 are excessive in S content, Ca content, REM content, O content, (Mg, Zr) the number of S particles is small, austenite grains are coarse, and HAZ toughness is average Both the value and the minimum value cannot satisfy the target value.

Steel No. No. 47 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. 49, Cu content, steel material No. 49. In No. 50, since the Cr content, the Nb content, and the V content exceed the upper limit, even if the austenite grains are fine, the HAZ toughness cannot satisfy the target value for both the average value and the minimum value.

According to the high-strength steel for welding 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 is extremely large.
Examples of the “welding steel material” by the high-strength steel for welding in the present invention include, for example, JIS G3106 “rolled steel material for welded structure”, JIS G3115 “steel plate for pressure vessel”, JIS G3126 “carbon steel plate for low temperature pressure vessel”. It corresponds to.

Claims (3)

% By mass
C: 0.05% or more, less than 0.10%,
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,
Zr: 0.0010% or more, 0.0050% or less,
N: 0.0020% or more, 0.0050% or less,
O: 0.0007% or more, containing 0.0020% or less,
Si: less than 0.10%,
P: 0.01% or less,
Ca: 0.0005% or less,
REM: 0.0005% or less,
Cu: 1.0% or less,
Ni: 1.5% or less,
Cr: 0.6% or less,
Mo: 0.40% or less,
Nb: 0.020% or less,
V: limited to 0.060% or less,
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 1.00 or more and 3.00 or less,
(Mg, Zr) S having a particle size of 0.015 μm or more and 0.2 μm or less and the ratio of Mg in the total of Mg and Zr is 70% or more and 90% or less in terms of atomic%. A high-strength steel for welding, comprising 0 × 10 ⁴ or more and 3.0 × 10 ⁵ or less.
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,
Cu: 0.5% or less,
Cr: 0.3% or less,
Mo: The high-tensile steel for welding according to claim 1, characterized by being limited to 0.10% or less. 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 high-strength steel for welding according to claim 1 or 2, wherein