JPH093600A - Weld heat-affected zone steel material with excellent toughness for welded structures - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a low temperature steel material with excellent toughness in the weld heat affected zone. [Constitution] Molten steel is deoxidized with Ti and Al, and further 0.00
By adding Mg in an amount of 10% or less, the particle size becomes 0.
The composite oxide mainly composed of Al-TiMg having a particle size of 01 to 1.0 μm has a particle number of 1 × 10 ⁴ to 2 × 10 ⁵ particles / mm ^2.
A low-temperature steel material characterized by being uniformly and finely dispersed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding heat affected zone (hereinafter referred to as "H") used in marine structures, ships, storage tanks, middle- and high-rise buildings, etc.
(Referred to as AZ), which has excellent toughness.

[0002]

2. Description of the Related Art In recent years, the demand for material properties of welded structural steel used for large-scale structures used at low temperatures such as marine structures, storage tanks, and ships has become increasingly severe, and damage caused by destruction has been increased. The demand for HAZ toughness is increasing, as is the toughness of steel itself, due to the size of the steel and the degree of social unrest.

Further, in constructing such a structure, in order to promote the efficiency of welding, application of a large heat input welding method represented by a flux-copper backing welding method, an electrogas arc welding method and the like is advanced. Has been.

In response to this, the HAZ of steel during high heat input welding
Many proposals have focused on toughness.

For example, as disclosed in JP-B-55-26164, there is a method of reducing the austenite grains of HAZ and improving the toughness by securing fine Ti nitride in the steel. Also, Japanese Patent Application Laid-Open No.
Japanese Patent No. 64614 proposes a method of utilizing the composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite to improve the toughness of HAZ.

However, since Ti nitride almost forms a solid solution in the vicinity of the boundary (referred to as a weld bond portion) between the HAZ and the weld metal whose maximum temperature reaches more than 1400 ° C., the effect of suppressing deterioration of toughness decreases. However, it is difficult to meet the recent demands for strict steel material properties.

As a method of improving the toughness in the vicinity of the welded bond, steel containing Ti oxide is used in various fields such as thick plate and shaped steel. For example, in the field of thick plates, Japanese Patent Application Laid-Open Nos. 61-79745 and 62-103344.
As exemplified in the publication, a steel containing a Ti oxide is very effective in improving the toughness of a large heat input welded portion, and its application to a high-strength steel requiring low temperature toughness is promising. The principle is that fine ferrite is generated with Ti oxide, Ti nitride, and precipitates such as MnS as nuclei, and as a result, generation of coarse ferrite harmful to toughness is suppressed and deterioration of toughness can be prevented. . However, the number of such Ti oxides dispersed in steel cannot be so large. The cause is coarsening and agglomeration of Ti oxide, and if an attempt is made to increase the number of Ti nitrides,
Coarse Ti oxides of 5 μm or more, so-called inclusions increase. The inclusions having a thickness of 5 μm or more are harmful as a starting point of structural destruction and cause a decrease in toughness. Therefore, in order to further improve the HAZ toughness, it is necessary to utilize an oxide that does not cause coarsening or aggregation and is more finely dispersed than the Ti oxide.

Further, in the method disclosed in Japanese Patent Laid-Open No. 61-79745, the upper limit of the amount of Al is limited to a very small amount of 0.007% in order to easily form Ti oxide. .

When the amount of Al in the steel material is small, the toughness of the base material may decrease due to the above-mentioned coarsening of the oxide-based inclusions and the insufficient amount of AlN precipitates. In addition, when a steel plate having a small amount of Al is welded using a welding material that is normally used, the toughness of the weld metal may decrease.

[0010]

[Problems to be Solved by the Invention]
In order to improve the AZ characteristics, it does not coarsen like Ti oxide, and therefore does not serve as a starting point of fracture, and also serves as a nucleus site for precipitates such as Ti nitride and MnS, and austenite grain refinement and The object was to disperse an oxide capable of achieving excellent HAZ toughness by forming fine ferrite.

[0011]

In order to solve the above-mentioned problems, the present invention provides, by weight%, C: more than 0.09 to 0.20% Si: ≤ 0.50% Mn: 0.50. 1.7% P: <= 0.02% S: 0.002-0.010% Al: 0.005-0.020% Ti: 0.005-0.020% N: 0.0020-0.0060 % Mg: ≦ 0.0010%, the balance consisting of Fe and unavoidable impurities, and having a particle size of 0.01 to 1.0 μm and a particle number of 1 × 10 ^4.
As a first means, a welded structural steel material having excellent weld heat affected zone toughness, characterized in that it contains a composite oxide containing Al-Ti-Mg as a main component in an amount of up to 2 x 10 ⁵ pieces / mm ² . weight%
Then, C: more than 0.09 to 0.20% Si: ≤ 0.50% Mn: 0.50 to 1.7% P: ≤ 0.02% S: 0.002 to 0.010% Al: 0 0.005 to 0.020% Ti: 0.005 to 0.020% N: 0.0020 to 0.0060% Mg: ≤ 0.0010% as a basic component, and further Cu: ≤ 0.8% Ni: ≤ 1.0% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: 0.0005 to 0.0020% One or more kinds. Contained, the balance being Fe and unavoidable impurities, and having a particle size of 0.01 to 1.0 μm.
m, the number of particles is 1 × 10 ⁴ to 2 × 10 ⁵ particles / mm ² , and Al
Secondly, a welding steel material having excellent toughness in a heat-affected zone of a welding heat treatment, characterized by containing a complex oxide mainly composed of -Ti-Mg
Means.

[0012]

The present invention will be described in detail below. The present inventors have found that as a metallographic factor that improves HAZ toughness,
To achieve austenite grain refinement in a region heated to less than 400 ° C., and intragranular ferrite formation in a region heated to 1400 ° C. or more in the vicinity of a weld bond portion at the same time by using an oxide, Regarding these two items, it was found that the composite oxide mainly composed of Al-Ti-Mg is effective.

In order to make austenite finer, it is necessary to suppress austenite grain growth at high temperature. As a means for this, a method of pinning austenite grain boundaries with precipitates and stopping the movement of the grain boundaries can be considered. As one of the precipitates having such an action, it is generally considered that Ti nitride is effective. It is a well-known fact that the larger the number of precipitates, the smaller the austenite crystal grain size. Therefore, in order to refine austenite, it is effective to precipitate a large number of Ti nitrides. The composite oxide mainly composed of Al-Ti-Mg serves as a nucleation site for Ti nitride,
This contributes to the deposition of a large number of nitrides.

Regarding the formation of intragranular ferrite, the inventors of the present invention observed the structure of intragranular ferrite formed within austenite grains and investigated the particles contained in the intragranular ferrite. As a result, A
It has been found that a complex oxide mainly composed of 1-Ti-Mg and a complex of Ti nitride + MnS deposited thereon effectively act. The oxide is stable even when heated to a high temperature, and exists stably in steel without change even at 1400 ° C. or higher. Further, since Ti nitride + MnS precipitates as an oxide forming site mainly of Al—Ti—Mg in the subsequent cooling process, intragranular ferrite can be formed in the vicinity of the weld bond.

In order to obtain the above-mentioned metallic texture effect, it is necessary that the particle diameter of the oxide is 0.01 to 1.0 μm. According to the knowledge of the present inventors, the particle size is 0.
If it is less than 01 μm, the effect as Ti nitride precipitation nuclei is weak, and if it exceeds 1.0 μm, the oxide becomes more likely to become a starting point of fracture, resulting in a decrease in HAZ toughness.

Next, the number of composite oxides will be described. If the number of oxides is too small, sufficient Ti nitride and intragranular ferrite formation nuclei cannot be obtained during welding.
It is necessary to have ⁴ or more oxides / mm ² present. As the number of oxides increases, the number of Ti nitrides and intragranular ferrites increases and the HAZ toughness improves, but if excess oxides exceeding 2 × 10 ⁵ / mm ² are present, the HAZ part and the base metal Since the toughness is reduced, the upper limit of the number of oxides must be 2 × 10 ⁵ / mm ² .

The size and number of the oxides are measured as follows. An extract replica is prepared from a steel sheet as a base material, and the size and the number of the oxide are measured by observing at least 20 visual fields at a magnification of 10000 and an observation area of 1000 μm ² or more with an electron microscope. At this time, an extracted replica collected from any part from the surface part to the center part of the steel sheet may be used.

As a process for producing a steel material, the effect of the oxide is not affected even if it is normally rolled, controlled rolling, a combination of this and controlled cooling and tempering, and a combination of quenching and tempering. .

Next, the reasons for limiting the range of the basic components of the present invention will be described.

C is an effective component for improving the strength of steel, the lower limit of which is more than 0.09%, and excessive addition exceeding 0.20% significantly deteriorates the weldability of steel materials and the HAZ toughness at low temperatures. Therefore, the upper limit was made 0.20%.

Si is a component necessary for securing the strength of the base material, pre-deoxidizing, etc., but the upper limit was made 0.5% in order to prevent deterioration of toughness due to hardening of the HAZ.

Mn is 0.5% as a component that secures the strength and toughness of the base material and generates transformation nuclei of intragranular ferrite.
Although the above additions are necessary, the upper limit was made 1.7% within the allowable range of the toughness and crackability of the welded portion.

The smaller the content of P is, the more preferable it is. However, in order to reduce this industrially, it costs a great deal of money, so 0.020% was made the upper limit.

S is 0.00 as an element that produces MnS.
2% is required, but the upper limit was made 0.01% within the allowable range of the toughness and crackability of the welded portion.

In order to increase the number of oxides and to control the toughness of the weld metal, Al has a lower limit of 0.0.
05%. Further, when Al is present in a large amount, all the oxides become alumina and a composite oxide mainly composed of Al-Ti-Mg is not formed, so the upper limit is 0.020.
%.

Ti is Al-Ti-Mg composite oxide, Ti
Add 0.005% or more to form a nitride.
However, if the amount of solid solution Ti increases, the HAZ toughness decreases, so 0.020% was made the upper limit.

N is an extremely important element for the precipitation of Ti nitride, and if it is less than 0.002%, the amount of precipitation of Ti nitride will be insufficient and a sufficient amount of ferrite structure will not be obtained.
Further, since an increase in solute N causes a decrease in HAZ toughness, the upper limit was made 0.006%.

Mg plays a particularly important role in the present invention. As described in JP-A-61-79745, when the amount of Al in the molten steel increases, the oxide mainly becomes alumina, so that the Ti-based oxide is hard to be generated. However, according to the knowledge of the present inventors, when Mg is present in the molten steel, A
It was clarified that the alumina decreased and the Ti-based oxide increased and the Al-Ti-Mg composite oxide was generated even if the amount of 1 was large. However, if the amount of Mg is too large, the Ti-based oxide is reduced by Mg, and it becomes difficult to generate an Al-Ti-Mg composite oxide, so the upper limit is 10 ppm (0.00
10%).

Cu is effective for improving the strength of the steel material, but if it exceeds 0.8%, the HAZ toughness decreases, so 0.8% was made the upper limit.

Ni is effective in improving the strength and toughness of the steel material, but an increase in the amount of Ni increases the manufacturing cost, so the upper limit was made 1.5%.

Nb is an effective element for improving the strength and toughness of the base material by improving the hardenability, but excessive addition in the HAZ portion significantly lowers the toughness, so 0.03% is added. The upper limit was set.

Since V, Cr, and Mo have the same effect as Nb, 0.1%, 0.6%, and
The upper limit was 0.6%.

B is a grain boundary ferrite harmful to HAZ toughness,
0.0005% or more by suppressing the growth of the ferrite side plate and fixing the solid solution N of HAZ by the precipitation of BN.
002% or less.

[0034]

EXAMPLE A 50 kg steel was prototyped with the chemical composition shown in Table 1. 1 to 16 are steels of the present invention, and 17 to 22 are comparative steels. The prototype steel is melted in a converter and deoxidized during vacuum degassing. Before the addition of Ti, the dissolved oxygen of the molten steel is adjusted with Si, then Ti and Al are sequentially added to perform deoxidation, and further Mg is added, and then cast into a 280 mm thick slab by continuous casting, followed by heat rolling, It was manufactured as a steel plate having a plate thickness of 32 mm. The obtained steel sheet was subjected to 1 pass flux-copper backing welding (FCB welding).

Table 1 shows the chemical composition of the base material. Table 2 shows the number of oxide particles, the base material characteristics, and the HAZ toughness.
The Charpy value for HAZ toughness evaluation is from the fusion line to HAZ5 at -20 ° C and -40 ° C.
It is an average value obtained by conducting 9 tests at a site of mm.

[0036]

[Table 1] As is clear from Table 2, it is understood that the inventive steels 1 to 16 have superior HAZ toughness as compared with the comparative steel. That is, Al-Ti-Mg having a particle diameter of 0.01 to 1.0 μm
The number of particles of the composite oxide is 1 × 10 ⁴ to 2 × 10 ⁵ particles / mm
^In the range of ² , the toughness at -20 ° C and -40 ° C is extremely excellent. On the other hand, in the comparative steels, 17, 18
The HAZ toughness is inferior because the number of oxides is too small and the number of oxides of 19 and 20 is too much and exceeds the range. In No. 21, the amount of added Al was too large and the oxide did not become an Al-Ti-Mg composite oxide, which did not serve as a nucleation site for Ti nitride and the number of Ti nitrides was insufficient, resulting in a coarse austenite grain size. This is an example in which the HAZ toughness is deteriorated due to the formation of the HAZ. No. 22 is an example in which the number of oxides was in the range of 1 × 10 ⁴ to 2 × 10 ⁵ pieces / mm ² , but the amount of Al was small and the size was coarse, so that the HAZ toughness was lowered.

[0037]

[Table 2]

[0038]

INDUSTRIAL APPLICABILITY The present invention provides a steel sheet which is used at low temperature and which satisfies the severe toughness requirement for destruction of marine structures, storage tanks, ships, etc., and the effect brought to this kind of industrial field is extremely high. The contribution to society is very large because of the safety of the structure.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Otani No. 1 Nishinoshu, Oita-shi, Oita-shi Nippon Steel Corporation Oita Works

Claims (2)

[Claims] 1. By weight%, C: more than 0.09 to 0.20% Si: ≤ 0.50% Mn: 0.50 to 1.7% P: ≤ 0.02% S: 0.002 0.010% Al: 0.005-0.020% Ti: 0.005-0.020% N: 0.0020-0.0060% Mg: ≤ 0.0010%, with the balance being Fe and unavoidable. Consisting of impurities, having a particle size of 0.01 to 1.0 μm and a particle number of 1 × 10 ⁴
~ 2 × 10 ⁵ pieces / mm ² containing a complex oxide mainly composed of Al-Ti-Mg, a steel material for welded structure having excellent toughness in the weld heat affected zone 2. By weight%, C: more than 0.09 to 0.20% Si: ≤0.50% Mn: 0.50 to 1.7% P: ≤0.02% S: 0.002 0.010% Al: 0.005-0.020% Ti: 0.005-0.020% N: 0.0020-0.0060% Mg: ≤ 0.0010% as a basic component, and further Cu: ≤ 0.8% Ni: ≦ 1.0% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: 0.0005 to 0.0020% One or two or more kinds are contained, the balance is Fe and unavoidable impurities, and the particle size is 0.01 to 1.0 μm.
m, the number of particles is 1 × 10 ⁴ to 2 × 10 ⁵ particles / mm ² , and Al
A steel material for welded structure having excellent toughness in the heat-affected zone of a welded heat, which contains a composite oxide mainly composed of -Ti-Mg.