JP3224677B2 - Low temperature steel for welding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat affected zone (hereinafter referred to as HAZ) used for ships, marine structures, storage tanks and the like.
Low-temperature steel for welding with excellent toughness.

[0002]

2. Description of the Related Art In recent years, the demand for material properties of welded structural steel used for large structures such as ships, marine structures, storage tanks and the like has been increasing strictly.
The requirements for the toughness of the HAZ are also increasing. In addition, application of large heat input welding is being promoted along with a change in welding methods.

[0003] For example, the required value of HAZ toughness for steel for tanks storing liquefied gas at -50 ° C is -50.
Considering the CTOD value at 80 ° C. and the difference between the CTOD characteristic and the Charpy characteristic, the Charpy HAZ at −80 ° C.
Toughness may be required.

In addition, marine structures, icebreakers, and the like used in the Arctic Ocean are also required to guarantee toughness in a use environment at -60 ° C. In recent years, the requirements for oil and natural gas development projects off the coast of Sakhalin, which are being developed, have become even more demanding. Toughness assurance at -80 ° C in Charpy tests and CTOD (Crack Tip Opening Di
In the test of (placement), assurance of toughness at -50 ° C is required.

Further, when constructing such a structure, it is desired to apply a large heat input welding method such as a single-sided one-pass welding method typified by flux-copper backing welding in order to promote more efficient welding. ing.

Conventionally, the demand for the CTOD characteristic has been limited to a portion to which small-to-medium heat input welding is applied, and a method for improving the CTOD characteristic is disclosed in, for example, Japanese Patent Publication No. 4-14179 and Japanese Patent Application Laid-Open No. 4-116135. It was sufficient to control the state of formation of island martensite, which governs the CTOD characteristics, by regulating the components as disclosed in the gazette. However, in recent years, the application of large heat input welding has been promoted, and in that case, controlling only the island-like martensite is not sufficient.

[0007] In response to this, HAZ of steel material at the time of large heat input welding
There have been many proposals focusing on toughness.

[0008] For example, as disclosed in Japanese Patent Publication No. 55-26164, there is a method of reducing the austenite grains of HAZ and improving the toughness by securing fine TiN in steel. Also, JP-A-3-2646
No. 14 proposes a method for improving the toughness of HAZ by utilizing a composite precipitate of TiN and MnS as a transformation nucleus of ferrite.

However, TiN almost completely forms a solid solution in the vicinity of a boundary (referred to as a weld bond portion) with a weld metal having a maximum temperature exceeding 1400 ° C. of the HAZ, so that the effect of suppressing the deterioration of toughness is reduced. There is a problem, and it is difficult to achieve the recent strict requirements for steel material properties.

As a method for improving the toughness in the vicinity of the weld bond, steels containing Ti oxide are used in various fields such as thick plates and section steels. For example, in the field of thick plates, Japanese Patent Application Laid-Open Nos. 61-79745 and 62-103344.
As exemplified in the publication, steel containing Ti oxide is very effective for improving the toughness of a large heat input weld, and its application to high-strength steel requiring low-temperature toughness is promising. The principle is that fine ferrite is generated using Ti oxides and precipitates such as TiN and MnS as nuclei. As a result, generation of coarse ferrite harmful to toughness is suppressed, and deterioration of toughness can be prevented. However, such Ti
Oxides cannot be dispersed too much in the steel. The cause is the coarsening and aggregation of Ti oxides. If the number of Ti oxides is to be increased, 5 μm
The above coarse Ti oxides, so-called inclusions, increase. The inclusions having a size of 5 μm or more are harmful as starting points 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 which is less likely to be coarsened and aggregated and which is more finely dispersed than a Ti oxide.

[0011]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. 62-1033
In order to further improve the HAZ characteristics as compared with the conventional method such as that disclosed in Japanese Patent Application Laid-Open Publication No. 44-44, it does not become coarse as in the case of Ti oxide, and therefore does not become a starting point of destruction. Accordingly, an object of the present invention is to disperse an oxide capable of realizing excellent HAZ toughness by austenite grain refinement and generation of fine ferrite.

[0012]

The present invention provides the following inventions for solving the above-mentioned problems . (1) In weight%, C: 0.03 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: 0.0010 to 0.0050 % Al: 0.004 to 0.020% Ti: 0.005 to 0.025% N: 0.0020 to 0.0060%, the balance being Fe and unavoidable impurities, and a particle size of 0 .01 to 1.0 μm, the number of particles is 1 × 10 ⁶
~ 5 × 10 ⁸ / mm ³ , Ti composition ratio is 5% or more,
Al composition ratio is 95% or less, 95% or more of Ti and Al
A low-temperature steel for welding , comprising the composite oxide described above . (2) in wt%, Cu: ≦ 1.0% Ni : the invention which is characterized in that it contains ≦ 3.8% of one or
The low-temperature steel for welding according to (1) . (3) By weight%, Nb: ≤ 0.050% V: ≤ 0.1% Cr: ≤ 0.6% Mo: ≤ 0.6% B: ≤ 0.0002 to 0.0020% or Claims characterized by containing two or more types
Item 4. The low-temperature steel for welding according to item 1 or 2.

[0013]

Hereinafter, the present invention will be described in detail. The present inventors, as a metal composition factor to improve the HAZ toughness,
(1) Austenitic grain refinement in a region heated to less than 1400 ° C. and (2) Intragranular ferrite generation in a region heated to 1400 ° C. or more in the vicinity of a weld bond portion are simultaneously achieved by using an oxide. It was investigated.

Regarding the above item (1), it is necessary to suppress austenite grain growth at a high temperature in order to reduce austenite grain size. 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, TiN is generally considered to be 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, Ti
It is effective to deposit a large amount of N. From such a viewpoint, the present inventors have observed in detail the TiN precipitated in the steel, and found that TiN precipitated as oxide nucleation sites is frequently present. Such an oxide contains Ti and A when the content of Ti is 5% or more.
1 and a particle size of 0.01 to 0.1.
It was 1 μm. The composite oxide of Ti and Al is TiN
And N conformity are good, so that TiN can preferentially generate nuclei with an oxide as a site. That is, the presence of the composite oxide of Ti and Al having a particle diameter of 0.01 to 0.1 μm in the steel makes the site where TiN precipitates as compared with the case where the composite oxide of Ti and Al does not exist. Is increased, and the number of precipitated TiN is increased. As a result, austenite grains pinned by a large number of TiN can be refined.

Regarding the above item (2), the present inventors have observed the structure of intragranular ferrite generated in austenite grains and investigated the particles contained in the intragranular ferrite. As a result, 0.1 to 1.
It has been found that a composite oxide of Ti and Al having a size of 0 μm and a composite of TiN + MnS deposited thereon work effectively. The oxide is stable even when heated to a high temperature, and exists stably in steel without change even at 1400 ° C. or higher. In the subsequent cooling process, TiN + MnS precipitates a composite oxide of Ti and Al as nucleation sites, so that intragranular ferrite can be generated in the vicinity of the weld bond.

From the above findings, in order to reduce the size of austenite grains in a region heated to less than 1400 ° C. and to generate intragranular ferrite in a region heated to 1400 ° C. or more in the vicinity of a weld bond, it is necessary to use particles. Diameter 0.01 ~
It is necessary that 1.0 μm of a composite oxide of Ti and Al is present in the steel. According to the findings of the present inventors, when the particle size is less than 0.01 μm, the effect as a TiN precipitation nucleus is weak, and when the particle size exceeds 1.0 μm, there is a high possibility that the oxide becomes a starting point of fracture. It becomes, which leads to deterioration of HAZ toughness.

Next, the number of composite oxides of Ti and Al will be described. If the number of oxides is too small, sufficient nuclei for forming TiN and intragranular ferrite at the time of welding cannot be obtained. Therefore, it is necessary to contain oxides of 1 × 10 ⁶ / mm ³ or more. As the number of oxides increases, T
Although the number of iN and intragranular ferrite increases and the HAZ toughness improves, the presence of an excess oxide exceeding 5 × 10 ⁸ / mm ³ causes a decrease in the toughness of the HAZ portion and the base material. The upper limit is 5 × 10 ⁸ pieces / mm
Must be ³ .

The size and the number of the oxide are measured by preparing an extraction replica from a sample taken from a steel plate and observing 20 fields of view at 10,000 times with an electron microscope.

The composite oxide of Ti and Al is formed by adding Ti and Al after deoxidizing molten steel with an element having a lower deoxidizing power than Ti, such as Si and Mn, when deoxidizing molten steel. I do. This is called a primary oxide. Furthermore, during casting and solidification, a composite oxide of Ti and Al is generated as the temperature of the molten steel decreases. This is called a secondary oxide. In the present invention, either a primary oxide or a secondary oxide may be used.

Further, in order to control the composition, number and size of the oxides under predetermined conditions, a deoxidizing method in the steel making process is important. As an appropriate deoxidation method, the amount of dissolved oxygen [%
O] in molten steel adjusted to be 20 to 80 ppm, and after adding Ti so that the final content becomes a predetermined component value and deoxidizing, the final content becomes a predetermined component value% within 5 minutes.
Is added, and then other elements such as Si and Mn, which are insufficient for the final component, are added to adjust the final component. The introduction of Al after deoxidation of Ti is because the introduction of Al partially reduces the Ti oxide and stably generates a composite oxide of Ti and Al, and the amount of dissolved oxygen decreases by the addition of Al. Therefore, the growth of oxides is suppressed, the oxides are miniaturized, and it is difficult to float, so that the required number is secured. Further, as shown in FIG. 1, if the amount of dissolved oxygen [% O] is too small, the absolute amount of the oxide decreases and the number becomes insufficient. If the amount of dissolved oxygen [% O] is too large, the oxide becomes too small. Coarsening and excessive number of pieces.

The effect of the oxide is not affected by the process of producing steel material, which is usually performed as-rolled, controlled rolling, a combination of controlled rolling and tempering, or 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, with the lower limit being 0.03%, and an excessive addition exceeding 0.09% significantly lowers the weldability of steel materials and HAZ toughness at low temperatures. Therefore, the upper limit was set to 0.18%.

Si is a component necessary for ensuring the strength of the base material, preliminarily deoxidizing, etc., but the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ.

Mn is 0.4% as a component for ensuring the strength and toughness of the base material and for forming transformation nuclei of intragranular ferrite.
Although the above addition is necessary, the upper limit is set to 2.0% within an allowable range such as the toughness and cracking property of the welded portion.

N is an extremely important element for the precipitation of TiN, and if it is less than 0.0020%, the amount of TiN deposited becomes insufficient, and a sufficient amount of ferrite structure cannot be obtained. Further, since the increase of solid solution N causes a decrease in HAZ toughness, the upper limit is made 0.0060%.

Although Cu is effective for improving the strength of the steel material, if it exceeds 1.0%, the HAZ toughness is reduced. Therefore, the upper limit is set to 1.0%.

Although Ni is effective for improving the strength and toughness of the steel material, the upper limit is set to 3.8% because an increase in the Ni content increases the production cost.

Nb is an effective element for improving the strength and toughness of the base material by improving the hardenability. However, in the HAZ portion, excessive addition significantly reduces toughness, so that 0.05% is required. The upper limit was set.

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

B is a grain boundary ferrite harmful to HAZ toughness,
0.0002% or more from the suppression of the growth of the ferrite side plate and the fixation of the solute N in the HAZ by the precipitation of BN.
002% or less.

[0032]

EXAMPLES 40 kg steel and 50 kg steel were experimentally produced with the chemical components shown in Tables 1 and 2. 1 to 13 are steels of the present invention, 1
Nos. 4 to are comparative steels. The prototype steel is melted in a converter and deoxidized during vacuum degassing. Before the introduction of Ti, the dissolved oxygen of the molten steel is adjusted with Si, and then Ti and Al are sequentially added to perform deoxidation, cast into a 280 mm thick slab by continuous casting, and then subjected to heat rolling to form a 32 mm thick steel sheet. Manufactured. The obtained steel sheet was subjected to one-pass flux-copper backing welding (FCB welding). The heat input is 45 kJ / cm ² .

Table 1 shows the chemical composition of the base material and the particle diameter and number of oxides. Table 3 shows the properties of the base material and the toughness of the HAZ. The Charpy value for HAZ toughness evaluation is
Nine tests were performed at a HAZ 5 mm from the fusion line, and the average value was obtained.

As is clear from Table 3, the steels of the present invention Nos. 1 to 13 have excellent HAZ toughness as compared with the comparative steels. That is, the particle diameter is 0.005 to 1.0 μm,
Ti having a Ti composition ratio of 5% or more and an Al composition ratio of 95% or less
The number of particles of the composite oxide of Al and Al is 1 × 10 ^{6 to} 5 × 10
It is in the range of ⁸ pieces / mm ³ and the toughness at −40 ° C. and −60 ° C. is extremely excellent. On the other hand, in the comparative steel, 1
Nos. 4 and 15 have 16,1 due to the small number of oxides.
7 is H because the number of oxides is too large beyond the range.
AZ toughness is poor. In No. 18, the Al content in the oxide exceeded 95% due to the excessive amount of Al added, so that TiN nucleation sites were not formed, the TiN number was insufficient, the austenite grain size was coarsened, and the HAZ toughness was increased. This is an example in which is decreased. 19 is 1 × 10 ^{6 to} 5 × 10 ⁸ oxides /
Although it is in the range of mm ^3, the HAZ toughness is reduced due to the coarseness of the size.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

According to the present invention, a steel plate which is used at low temperature and satisfies the strict toughness requirement for breaking of ships, marine structures, storage tanks, etc. is provided, and the effect brought to this kind of industrial field is extremely large. In addition, the contribution to society is very large in terms of the stability of the structure.

[Brief description of the drawings]

FIG. 1 shows the relationship between the amount of dissolved oxygen and the number of oxides having a particle diameter of 0.01 to 1.0 μm, and shows that an appropriate number of oxides is generated in a certain range of the amount of dissolved oxygen. FIG.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Seiji Nishimura 1 Nishinosu, Oita-shi, Nippon Steel Corporation Oita Works (56) References JP-A-4-103742 (JP, A) JP-A Sho 61-117245 (JP, A) JP-A-7-242985 (JP, A) JP-B-6-21321 (JP, B2) JP-B 5-69902 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] C: 0.03 to 0.18% Si: ≤0.50% Mn: 0.40 to 2.0% P: ≤0.02% S: 0.0010 to 0% by weight% 0.0050% Al: 0.004 to 0.020% Ti: 0.005 to 0.025% N: 0.0020 to 0.0060%, the balance being Fe and unavoidable impurities, and particle size Is 0.01 to 1.0 μm and the number of particles is 1 × 10 ⁶
~ 5 × 10 ⁸ / mm ³ , Ti composition ratio is 5% or more,
Al composition ratio is 95% or less, 95% or more of Ti and Al
A low-temperature steel for welding , comprising the composite oxide described above . 2. A weight%, Cu: ≦ 1.0% Ni : claim 1, characterized in that it contains ≦ 3.8% of one or
The low temperature steel for welding as described . In wherein wt%, Nb: ≦ 0.050% V : ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: ≦ 0.0002~0.0020% of 1 Claims characterized by containing a species or two or more species
Item 4. The low-temperature steel for welding according to item 1 or 2.