JP2001342537A - Steel material excellent in toughness of welding heat- affected zone and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a steel material with an excellent HAZ toughness in which an austenite particle enlargement by heating at a high temperature for a long time is greatly suppressed even in welding with a large amount of heat input of 200 kJ/cm2 or more. SOLUTION: The steel material excellent in heat-affected zone toughness, is characterized in that in a steel having required components, oxide particles in circular equivalent diameter of 0.005-2.0 μm, preferably 0.1-2.0 μm, are contained, and oxide particles having at least Ca and Al, or Ca, Al and Mg as the composition, wherein the mass ratio of elements excepting O, is Ca: 5% or above and Al: 5% or above, or Ca: 5% above, Al: 5% above and Mg: 1% or above, are contained in 100-5,000 pieces/mm2.

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 for ships, offshore structures, middle and high rise buildings and the like.
AZ) and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, ships, marine structures, middle and high-rise buildings,
Demands on the material properties of welding steel used for large structures such as bridges are increasing. Furthermore, when building such a structure, to promote more efficient welding,
It is desired to apply a large heat input welding method typified by a flux-copper backing welding method, an electrogas welding method, an electroslag welding method, etc., as well as the toughness of the steel material itself, as well as the demands on the toughness of the HAZ. Are also becoming more severe.

There have been many proposals focusing on the HAZ toughness of steel during large heat input welding.

[0004] For example, as disclosed in Japanese Patent Publication No. 55-26164, there is a method of reducing austenite grains of HAZ and improving toughness by securing fine Ti nitride in steel. Also, Japanese Patent Application Laid-Open No. 3-2
Japanese Patent No. 64614 proposes a method of improving the toughness of HAZ by utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite.

[0005] However, Ti nitride almost completely forms a solid solution near the boundary (referred to as a weld bond portion) with the weld metal having a maximum temperature exceeding 1400 ° C. of the HAZ, so that the effect of improving the toughness is reduced. Therefore, 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 in improving the toughness of a large heat input weld, and its application to high-strength steel is promising. This principle is based on the fact that Ti nitrides, MnS, etc., are precipitated while the temperature is lowered after welding, and fine ferrite is generated using the Ti oxides, which are stable even at the melting point of steel, as sites. The formation of harmful coarse ferrite is suppressed, and the deterioration of toughness can be prevented. However, the number of such Ti oxides dispersed in steel cannot be so large. This is due to coarsening and aggregation of Ti oxides. If the number of Ti oxides is to be increased, coarse Ti oxides of 5 μm or more, so-called inclusions, will increase. The inclusions having a size of 5 μm or more are harmful as a starting point of the destruction of the structure 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.

As a method for dispersing such Ti oxides in steel, there are many methods in which Ti is added to molten steel substantially containing no strong deoxidizing element such as Al. However, it is difficult to control the number and dispersity of Ti oxides in steel simply by adding Ti to molten steel, and further control the number and dispersity of precipitates such as TiN and MnS. It is also difficult. As a result, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, Ti
Problems such as insufficient number of oxides and variation in toughness in the thickness direction of the thick plate are observed.

Further, in the method disclosed in Japanese Patent Application Laid-Open No. 61-79745 or the like, the upper limit of the amount of Al is limited to a very small amount of 0.007% in order to easily form a Ti oxide. . If the amount of Al in the steel is small,
There may be a case where the toughness of the base material is reduced due to a cause such as an insufficient amount of N precipitates. Further, when a steel sheet having a small amount of Al is welded using a commonly used welding material, the toughness of the weld metal may be reduced.

To solve such a problem, Japanese Patent Laid-Open No. 6-29
No. 3937 and JP-A-6-293937, Ti formed by adding Al immediately after adding Ti
-A technology utilizing an Al composite oxide has been proposed. With this technology, large heat input welding HAZ toughness can be greatly improved, but in recent years, in the shipbuilding industry and the construction industry, it is 200 kJ / cm or more, and when it is large, it is 100 kJ / cm.
A further increase in welding heat input of as much as 0 kJ / cm is in progress, and a steel material having even higher HAZ toughness is required. At this time, it is particularly necessary to improve the toughness in the vicinity of the weld fusion part.

[0010]

SUMMARY OF THE INVENTION The present invention provides an excellent HAZ toughness by further suppressing the austenite grain coarsening when heated to a high temperature for a long time even during the welding with the super-high heat input as described above. It is an object of the present invention to provide a steel material having excellent toughness of a weld heat-affected zone and a method of manufacturing the same.

[0011]

The present invention has been found to solve the above-mentioned problems by dispersing Ca, Al or fine oxides of Ca, Al, Mg in steel having a specific steel component. Thus, the present invention has been completed.

The gist of the present invention is as follows.

(1) In mass%, C: 0.03-0.
18%, Si: ≤ 0.5%, Mn: 0.4 to 2.0%,
P: ≦ 0.02%, S: ≦ 0.02%, Al: 0.
005 to 0.04%, Ti: 0.005 to 0.03%,
Ca: 0.0005 to 0.003%, N: 0.000
5 to 0.006%, O: 0.0005 to 0.006%
, The balance being steel consisting of Fe and unavoidable impurities, and having a circle equivalent diameter of 0.005 to 2.0 μm in the steel.
m of oxide particles in a number density per unit area of 100 to
5,000 particles / mm ^2, and the composition of the oxide particles is composed of at least elements of Ca, Al and O, and the element excluding O is mass%, and Ca is 5% or more and Al is 5% or more. A steel material with excellent toughness in the heat-affected zone of a weld, characterized in that:

(2) C: 0.03-0.
18%, Si: ≤ 0.5%, Mn: 0.4 to 2.0%,
P: ≦ 0.02%, S: ≦ 0.02%, Al: 0.
005 to 0.04%, Ti: 0.005 to 0.03%,
Ca: 0.0005-0.003%, Mg: 0.000
1 to 0.002%, N: 0.0005 to 0.006
%, O: 0.0005 to 0.006%, the balance being steel consisting of Fe and unavoidable impurities, and oxide steel particles having a circle equivalent diameter of 0.005 to 2.0 μm in the steel. 100-5000 pieces / mm in number density per area
² containing oxide particles having a composition of at least Ca, A
1, heat-affected zone toughness characterized in that the element excluding O is represented by mass%, and Ca is 5% or more, Al is 5% or more, and Mg is 1% or more. Excellent steel material.

(3) In mass%, Cu: ≦ 1.0%, N
i: ≦ 1.5%, Nb: ≦ 0.03%, V: ≦ 0.1
%, Cr: ≦ 0.6%. Mo: ≦ 0.6%, REM: ≦
The steel material having excellent toughness of the weld heat affected zone according to the above (1) or (2), containing 0.05%.

(4) The weld heat-affected zone toughness according to any one of (1) to (3), wherein the composition of the oxide particles further contains 1% by mass or more of S. Excellent steel material.

(5) The weld heat-affected zone toughness according to any one of (2) to (4), wherein the composition of the oxide particles further contains 1% or more by mass of Mg. Excellent steel material.

(6) The steel further comprises, by mass%, B
: 0.0005 to 0.003%, EN = (%
N) -0.292 (% Ti) -1.292 (% B) In the equivalent equation, 0 ≦ EN ≦ 0.002 is satisfied. Steel with excellent toughness in the heat affected zone of welding.

(7) The oxide particles have a circle equivalent diameter of 0.1.
(1) to (1) to 2.0 μm.
(6) The steel material excellent in toughness of the weld heat-affected zone according to any of (6).

(8) In the secondary refining process in a reduced-pressure atmosphere, Ti, Al, and Ca are added to molten steel having an oxygen concentration of 80 ppm or less by T
i, Al, and Ca are added in this order to obtain a molten steel having a chemical component and an oxide according to any one of the above (1) to (7),
A method for producing a steel material having excellent toughness in a heat-affected zone of a weld, characterized by casting this into a slab.

(9) When the oxygen concentration in the molten steel in the secondary refining step is more than 80 ppm to 200 ppm or less, the oxygen concentration in the molten steel is reduced to 80 ppm or less by adding Al or an Al alloy. The method for producing a steel material having excellent toughness of a weld heat affected zone according to the above (8).

(10) When the oxygen concentration in the molten steel in the secondary refining step is more than 200 ppm, C is added in a reduced pressure atmosphere and the oxygen concentration in the molten steel is reduced to 20 by C deoxidation.
The method for producing a steel material having excellent toughness of a weld heat affected zone according to the above (9), wherein the content is set to 0 ppm or less.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The present inventors have studied the use of oxides to achieve reheating austenite grain refinement in a HAZ region heated to 1400 ° C. or higher as a metal structure factor for improving HAZ toughness.

In order to reduce the size of the reheated austenite grains, it is necessary to suppress the growth of austenite grains at a high temperature. The most effective method is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. As one of the dispersed particles having such an effect, a nitride and an oxide of Ti have been conventionally considered to be effective. However, as described above, since the proportion of solid solution of Ti nitride at a high temperature of 1400 ° C. or more increases, coarse particles are partially generated to reduce the pinning effect. On the other hand, it is necessary to utilize oxides stable at high temperatures as pinning particles.

Furthermore, since the vicinity of the stable oxide becomes a non-uniform deformation region at the time of processing, it becomes a preferential nucleation site for recrystallization together with the γ grain boundary, and has the effect of promoting the miniaturization of recrystallized γ.

The effect of the dispersed particles on the pinning of the crystal grain boundaries increases as the volume ratio of the dispersed particles increases and as the diameter of one particle increases. However, since the volume fraction of dispersed particles has an upper limit depending on the concentration of the elements constituting the particles contained in the steel, if the volume fraction is assumed to be constant, a smaller particle diameter is more effective for pinning. . From such a viewpoint, the present inventors have conducted various studies to increase the volume fraction of the oxide and to obtain an appropriate particle size.

One way to increase the volume fraction of oxides is to increase the amount of oxygen. However, an increase in the amount of oxygen causes the formation of a large number of coarse oxides that are harmful to the material. It is not an effective means. Therefore, the present inventors have studied the use of an element having a small solubility product with oxygen in order to make maximum use of oxygen. Al is generally used as a material having a small solubility product with oxygen, that is, as a strongly deoxidizing element. However, Al alone is not sufficient to sufficiently utilize oxygen, and further requires a deoxidizing element stronger than Al. As a result of various studies, it is necessary to use Ca that is widely used in the deoxidizing process of steel. Was found to be effective. Since Ca has a small solubility product with oxygen, Ca can produce a larger amount of oxide than Al for the same amount of oxygen. As a result of various experiments using Ca as a deoxidizing element, the composition of the oxide particles generated in the steel contained 5% or more of Ca and 5% or more of Al, and thus the volume fraction of the oxide, It has been found that it is possible to increase the amount of oxide. On the basis of this result, the composition of the oxide particles contained in the steel was changed to include at least Ca, Al, O, and O
The elements excluding are set to have a mass ratio of Ca of 5% or more and Al of 5% or more.

It has also been found that the use of Mg together with Ca is also effective in producing a large number of oxides. Although Mg is not as effective as Ca, it is a stronger deoxidizing element than Al and has a small solubility product with oxygen. Therefore, it is possible to further increase the number of oxides by combining Mg with Ca and using it for deoxidation. The inventors conducted an experiment using Ca as a deoxidizing element, and as a result, as a composition of oxide particles generated in steel, Ca
Was found to be 5% or more and Mg to be 1% or more, thereby making it possible to further increase the volume fraction of the oxide, that is, the amount of the oxide. Based on this result, the composition of the oxide particles contained in the steel was changed to at least Ca, Al, Mg,
The element containing O and excluding O has a mass ratio of Ca of 5% or more,
Al was set to 5% or more, and Mg was set to 1% or more.

Furthermore, the present inventors have found that sulfides such as CaS and MgS precipitate around the oxide,
They have found that it is possible to further increase the volume fraction by combining oxides and sulfides. Based on this result, the composition of the particles contained in the steel was changed to at least Ca, A
Elements containing l, O, and S, but excluding O,
% Or more, Al is 5% or more, S is 1% or more, or an element containing at least Ca, Al, Mg, O, and S and excluding O is 5% or more of Ca and 5% or more of Al by mass ratio. , Mg
Was set to 1% or more, and S was set to 1% or more. In addition, it was confirmed that the effect of the present invention was not affected even if a deoxidizing element weaker than Al, such as Si, Mn, or Ti, which was inevitably mixed, was included.

Next, the size of the oxide particles effective for pinning will be described.

The effect of the dispersed particles on the pinning of the crystal grain boundaries increases as the volume fraction of the dispersed particles increases and as the diameter of one particle increases, but when the volume ratio of the particles is constant, the size of one oxide particle is reduced. It was considered that the smaller the number, the larger the number of particles and the larger the pinning effect, but the smaller the value, the smaller the proportion of the particles present at the grain boundaries, and thus considered that the effect was reduced. As a result of a detailed investigation of the austenite particle size when heated to a high temperature using test pieces with variously changed particle sizes, pinning has a particle size of 0.005 to 2.0 μm. I found that the effect was great. Furthermore, it has been found that the pinning force for stopping the movement of the austenite grain boundary is stronger as the size of the dispersed particles is larger, and among the particle sizes of 0.005 to 2.0 μm, the size of the particles of 0.1 to 2.0 μm is particularly large. They have found that it is effective. When it is smaller than 0.1 μm, the pinning effect gradually decreases, and when it is smaller than 0.005 μm, the pinning effect is hardly exhibited. Also,
Oxide particles larger than 2.0 μm have a pinning effect, but may be a starting point of brittle fracture, and are therefore unsuitable in the properties of steel materials. Based on this result, the required particle size was set at 0.
005 to 2.0 μm, among which 0.1 to 2.0 μm
m is effective.

Next, the number of pinning particles required for HAZ toughness was examined.

The greater the number of oxide particles, the finer the texture unit
HAZ toughness improves as the number of particles increases,
HAZ toughness required for steel materials is
It depends on the welding method. Particularly required characteristics
Large heat input welding with high strength shipbuilding steel
Required HAZ toughness, such as test temperature-
Satisfies absorption energy of 50 J or more at 40 ° C
First, as shown in FIG.
The number of oxide particles of 2.0 μm is 100 / mm^TwoNeed more
It was found that. However, as the number of particles increases,
However, the effect of improving toughness is reduced,
Higher numbers produce coarser particles that are detrimental to toughness
Considering that the possibility increases, the upper limit of the number of particles is 50.
00 pieces / mm ^TwoIs appropriate. That is, when the number of particles is 50
00 pieces / mm^TwoIf it exceeds, the particle spacing becomes small and heating
It is effective for refining austenite grains,
Charpy impact due to the reduced distance between fractures
Rather harmful to toughness typified by absorbed energy
I understood that. Therefore, the effective and required number of particles
100 to 5000 pieces / mm^TwoAnd

The size and the number of the oxide particles are measured, for example, in the following manner. 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. In the measurement of the size, for example, a circle-equivalent diameter is obtained based on a photograph of a particle. 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. If the particles can be properly observed, the observation magnification may be reduced.

Oxide particles are generated when deoxidizing molten steel. This is called a primary oxide. Furthermore, during casting and solidification, Ti-Al-Ca oxide 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.

The effects of the oxides are not affected by the process for producing the steel material, which is normally performed as-rolled, controlled-rolling, a combination thereof with controlled cooling and tempering, or a combination of quenching and tempering.

Further, the production conditions of such a slab were examined in detail. Steel production process: In converter → ladle → secondary refining → continuous casting, oxide inclusions remaining in the slab are:
In particular, the oxygen concentration of molten steel before the start of deoxidation in the secondary refining process is suppressed to 80 ppm or less, and the deoxidizing elements are Ti, Al,
It has been found that by adding Ca in the order, the average particle size is remarkably reduced and the number increases. In addition, this makes it possible that the composition of the finally remaining inclusions can be included as the composition of the oxide particles in the steel, as described above, in Ca: 3% or more and Al: 1% or more. I learned. Here, secondary refining refers to a process performed by a vacuum refining device or a refining device in an inert gas after converter refining. If the molten steel oxygen concentration before the start of deoxidation in the secondary refining step exceeds 80 ppm and / or the order of adding the deoxidizing element is not as described above, the average size of the oxide-based inclusions is relatively large, and The number does not reach the target level. That is, based on these results, the decarbonized molten steel was added to Si and Mn.
After the addition, the process is transferred to a secondary refining process such as a vacuum refining device or a refining device in an inert gas, where the oxygen concentration of molten steel is 80 p.
pm or less, successively add Ti, Al and Ca in this order, and if the molten steel oxygen concentration is higher than 200 ppm, add C such as coke
After performing deoxidation to reduce the oxygen concentration in the molten steel to 200 ppm or less, and further adding a small amount of Al or an Al alloy to reduce the oxygen concentration to 80 ppm or less, Ti, Al, Ca
, And when the oxygen concentration of molten steel is higher than 80 ppm and lower than 200 ppm, a small amount of Al or A
1 alloy is added to reduce the oxygen concentration to 80 ppm or less, and then T
It was found that by adding i, Al, and Ca in this order, in any case, a slab in which fine inclusions satisfying the above requirements were dispersed could be produced. Here, Ti, Al, C
With respect to a, it is added in the form of metal grains or metal wires composed of the respective deoxidizing elements and unavoidable impurities, or alloy grains or alloy wires containing the deoxidizing elements. It has been found that the addition of the element improves the yield of the deoxidizing element and further promotes the miniaturization of the oxide.

On the other hand, by dispersing the oxide in the steel in this way, the reheated austenite grains of the HAZ are extremely effectively refined by pinning, and the HAZ toughness is improved accordingly. As the grain size increases, the grain boundary area increases and the ability to form ferrite from the grain boundary increases. In very strict toughness requirements, the relatively coarse ferrite, especially at the corners of the grain boundary (grain boundary triple point), becomes the starting point. It has been found that controlling the improvement of toughness as a new problem. In other words, relatively coarse ferrite formed at such grain boundaries and grain boundary triple points is suppressed.
If it can be improved, the toughness can be greatly improved by overlapping with the effect of miniaturizing the HAZ structure.
The problem of the relatively coarse ferrite formed at the grain boundary and the grain boundary triple point was first discovered by making the HAZ structure of large heat input welding finer than ever with oxides. .

The present inventors have further studied in order to dramatically improve the effect of improving the toughness by making the HAZ structure finer. As a result, when the reheated austenite grains are refined by dispersing a large number of fine oxides, in order to suppress the growth of ferrite at the grain boundaries and grain boundary triple points during the formation process of the HAZ structure, B It has been found that the addition is extremely effective. Furthermore, as a result of a detailed investigation of the mechanism of the effect of the addition of B, the balance between B and N is important. , And significantly improved and stabilized the toughness of the fine-grained HAZ.

In the Ti-added steel, since the affinity between Ti and N is extremely large, the HAZ toughness is determined as EN = (% N)-as shown in FIG.
An equivalent of 0.292 (% Ti) -1.292 (% B) can be arranged well. When the equivalent value is in the range of 0 to 0.002, the effect of addition is the greatest and the toughness is greatly improved. I understood. When the equivalent value is less than 0, the effect of B is not recognized. On the other hand, when the equivalent value is more than 0.002, although the ferrite is refined, the HAZ toughness is greatly reduced by excess N.

Further, the basic component range of the present invention will be described.

The lower limit of C is 0.03% as an effective component for improving the strength of steel, and the excessive addition significantly lowers the weldability and HAZ toughness of the steel material. did.

Si is a component necessary for securing the strength of the base material, deoxidizing, and the like, but the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ. Preferably, 0.
03-0.5%.

Mn needs to be added in an amount of 0.4% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is 2.0% in the allowable range of the toughness and cracking property of the welded portion. And

The content of P is desirably as small as possible. However, in order to industrially reduce the content of P, a large cost is required. Therefore, the upper limit is set to 0.02%.

The lower the content of S, the better. However, since it requires a great deal of cost to reduce it industrially, the upper limit was made 0.02%.

Al is an important deoxidizing element, and the lower limit is set to 0.005%. Also, when Al is present in a large amount,
Since the surface quality of the slab deteriorates, the upper limit is set to 0.04%.

Ti is a deoxidizing element and is added in an amount of 0.005% or more in order to combine with N to form a Ti nitride to exert a certain effect on the heating γ and the grain refinement of HAZ. However, when the amount of solid solution Ti increases, the HAZ toughness decreases. Therefore, the upper limit is set to 0.03%.

Ca must be added in an amount of 0.0005% or more in order to form a Ca-based oxide. However, excessive addition produces coarse inclusions,
The upper limit was 03%.

Mg must be added in an amount of 0.0001% or more in order to generate an Mg-based oxide. However, excessive addition produces coarse inclusions,
The upper limit was 03%.

N has the effect of improving the HAZ toughness by being precipitated as TiN, so the lower limit was made 0.0005%. However, an increase in solid solution N causes a decrease in HAZ toughness, so the upper limit was made 0.006%.

O is contained in the molten steel when the steel is decarburized,
Although reduced by the subsequent deoxidation reaction, at least 0.0005% is required in the final slab stage for the purpose of effectively using oxide-based inclusions. On the other hand, an excessive content means that a coarse deoxidation product (oxide) is contained, which is not preferable in terms of steel material properties. Therefore, the upper limit was made 0.006%.

B is an element effective in suppressing the growth of ferrite generated at the heated austenite grain boundary in the presence of N, and is added at least 0.0005%. However, if added in a large amount, the toughness of the steel material deteriorates, so the upper limit was made 0.003%.

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 1.5% because an increase in the amount of Ni increases the production cost.

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

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

REM has deoxidizing power next to Ca in molten steel,
There is a function of assisting the formation of fine oxides by Ca, but if it is added excessively, the cost is larger than that of Ca, and coarse inclusions are formed to impair the toughness of the steel sheet and HAZ. 05%.

In the actual manufacturing process, the added elements are not necessarily included in the 100% molten steel. Therefore, it is necessary to add the elements in consideration of the yield. There is no particular limitation on the method of addition. Any method may be used as long as it can be contained in steel so as to satisfy the above conditions. However, as described above, T
i, Al, Ca and deoxidizing elements Mg and REM are:
By adding in the form of an alloy containing the deoxidizing element, the yield of the deoxidizing element is improved, and the miniaturization of oxides generated in molten steel is further promoted.

[0060]

EXAMPLES (Example 1) 50 chemical components shown in Table 1 were used.
A kilo steel was prototyped. 1 to 9 are steels of the present invention, and 10 to 17 are comparative steels. The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. Before the introduction of Ti, the dissolved oxygen of the molten steel is adjusted with Si, 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 obtain a 45 mm thick steel sheet. Manufactured. The obtained steel plate was subjected to one-pass SEGARC welding. The heat input is about 200 kJ / cm ² .

[0061]

[Table 1]

Table 2 shows the composition of the oxide particles and the particle diameter of the oxide particles.
It shows the number of particles of 005 to 2.0 μm, the rolling conditions of the steel sheet, the properties of the base material, and the toughness of HAZ. The Charpy value for HAZ toughness evaluation was HAZ1m from the fusion line.
Nine tests were performed at the site of m, and the average value was obtained.

[0063]

[Table 2]

As is clear from Table 2, the steels of the present invention Nos. 1 to 9 have excellent HAZ toughness as compared with the comparative steels. That is, since the particle diameter, the number of particles, and the amount of the composition of the oxide are within the range of the present invention, the HAZ toughness at −40 ° C. is extremely excellent. In addition, among the steels of the present invention, invention steels 3, 4, 5, 6, 8, 9, and oxide particles having an oxide particle diameter of 0.1 to 2.0 μm in the range of 100 to 5000 particles / mm ^2. Invention steels 2, 4, 5, 6, in which the composition Mg is 1% or more,
It can be seen that 7, 8, 9, and Invention Steels 3, 4, and 5 in which the S of the oxide particle composition is 1% or more have better HAZ toughness than the other steels of the invention.

On the other hand, all of Comparative Examples 10 to 17 showed low toughness of less than 40 J at -40 ° C. in the Charpy test. The reason for this is that in Comparative Example 9, the content of Ca, Mg, and the number of oxide particles, which are the oxide particle compositions, are out of the lower limits of the present invention. Is outside the lower limit of the present invention. Further, Comparative Examples 11 to 16 are because the number of oxide particles was outside the lower limit of the present invention. In Comparative Example 17, the Al content of the oxide particles and the number of the oxide particles were outside the lower limits of the present invention.

Example 2 The chemical components shown in Table 3
Prototypes of 0-60 kg steel were produced. 1 to 8 are steels of the present invention, 9 to 1
5 is a comparative steel. The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. Before the introduction of Ti, the dissolved oxygen of the molten steel is adjusted with Si, and then Ti, Al, and Ca are sequentially added to perform deoxidation, cast into a 280 mm thick slab by continuous casting, and then subjected to hot rolling to a 50 mm thick plate. Manufactured as steel sheet. The obtained steel sheet was subjected to one-pass electrogas welding. The heat input is about 280 kJ / cm ² .

[0067]

[Table 3]

Table 4 shows the average composition of the oxide particles, the number of particles having a particle size of 0.005 to 2.0 μm measured by an electron microscope, and EN = (% N) −0.292 (% Ti) −1. .29
2 (% B), the average diameter of austenite grains of the HAZ structure measured by a cutting method in 20 visual fields of a 100 times optical micrograph, and the maximum ferrite size (width) of austenite grain boundaries or grain boundary triple points; And HAZ toughness. The HAZ toughness value is an average value obtained by collecting nine test pieces from a bond of the steel sheet after welding at a site of 1 mm HAZ and performing a Charpy test at −40 ° C.

[0069]

[Table 4]

As is evident from Table 4, the steels of the present invention Nos. 1 to 8 have excellent HAZ toughness as compared with the comparative steels. That is, when the particle diameter is 0.005 to 2.0 μm and C
a, the number of oxide particles containing Al in a predetermined composition is 100 to
By being in the range of 5000 / mm ² , the austenite grain size of the HAZ structure is smaller than that of the comparative steel,
Further, the ferrite at the austenite grain boundary or the grain boundary triple point is also reduced due to the effect of B, and as a result, −
The Charpy absorbed energy value at 40 ° C. greatly exceeds the average of 50 J generally required from the viewpoint of the fracture mechanics of a steel structure, and it is clear that the HAZ toughness is extremely excellent. In addition, 1, 2, 4, 5, 7, and 8 have a particle diameter of 0.1.
The number of particles of 1 to 2.0 μm is also 100 particles / mm ² or more,
Compared with Nos. 3 and 6, the austenite grain size is relatively small and the Charpy absorbed energy is high.

On the other hand, all of Comparative Examples 9 to 14 showed low toughness of less than 50 J at -40 ° C. in Charpy test. The reason is that in 9 to 12, the chemical components are out of the range of the present invention, and the composition and number of the oxide particles are out of the range of the present invention. Is within the range of the present invention, but the EN equivalent is out of the range of the present invention. Comparative Example 15
The steel had a higher oxygen content in the steel than the other steels, and the number of oxide particles was outside the upper limit of the present invention.

Example 3 20 cast pieces having the chemical components shown in Table 5 were produced. Both are 2,000 mm wide, 28
It is a slab having a thickness of 0 mm, and is manufactured by continuous casting after RH vacuum refining after tapping from the converter. The control of the molten steel oxygen and the addition of the deoxidizing element were performed in the RH vacuum refining facility under the conditions shown in Table 6. In the 1/4 width direction of the manufactured slab,
The number of inclusions of 0.005 to 2.0 μm and the average composition thereof were examined. Thickness direction, 20mm below surface layer, 70mm (1
厚 thickness), 140 mm (１ ／ thickness), 210 mm (3 /
The measurement was performed at each of four locations (4 thicknesses), and the measurement results in the thickness direction were almost constant. Table 6 also shows the results of the investigation at 1/4 thickness as representatives. The inclusions were measured by mirror-polishing, revealing the inclusions by etching, observing at least 20 visual fields with a scanning electron microscope at a magnification of 3000 times,
The size and number of inclusions were measured. The composition of the inclusion was determined by using an energy dispersive spectrometer to confirm that it was an oxide. At the same time, using a replica collected from the same part, and using a transmission electron microscope to 10,000
Observation was made at 20 times or more at × magnification to verify the validity of the measurement results with a scanning microscope.

[0073]

[Table 5]

[0074]

[Table 6]

Steels A to H are within the scope of the present invention,
-U are comparative examples in which the components and / or production conditions are out of the range of the present invention.

In the case of steels A to C and F to H in which the initial molten steel oxygen before the secondary refining is 80 ppm or less,
Subsequently, Ti, Al, and Ca are added in this order, and in the case of steel E having a molten steel oxygen concentration higher than 200 ppm, C deoxidation is performed in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel to 200 pp.
m or less, and then a small amount of Al or Al alloy is added to reduce the oxygen concentration to 80 ppm or less.
1 and Ca in order, and the oxygen concentration of molten steel is 80 pp.
In the case of steel D higher than m and lower than 200 ppm, a small amount of Al or an Al alloy was added to reduce the oxygen concentration to 80 ppm or less, and then Ti, Al, and Ca were added in that order. Ti,
Basically, sponge Ti was used for Ti, regenerated Al was used for Al, and grains of Ca-Si alloy were used for Ca.
An e-Ti alloy was used. Regarding the steel of the present invention, as a result of control of molten steel oxygen before deoxidation in the secondary deoxidation step and control of the deoxidation order, in each case, the number of effective fine inclusions of 0.005 to 2.0 μm is 100 to 5000 particles / mm ² , and the average composition (excluding oxygen) is Ca ≧ 3%, A
l ≧ 1%. As a result, it was confirmed that slabs in which a large number of extremely fine oxides were dispersed were produced. Further, among these, it was also confirmed that the steels B, D, and G deoxidized using the Fe-Ti alloy had a particularly large number of inclusions and also had a good Ti yield.

On the other hand, as for the comparative steels, the values of the slab components Ti, Al and Ca of the steels I to L are out of the range of the present invention, and the molten steel oxygen content and the deoxidation order are the same as those of the present invention. Regardless, the number of fine oxides has not reached the target number. Further, although the molten steel oxygen before the deoxidation in the secondary deoxidation step was higher than the range of the present invention, the steels M and N were subjected to the next deoxidation without adjusting the oxygen amount, so that fine oxides were formed. Have not been built. Further, since the order of deoxidation of steels P to U was different from that of the present invention, the number of effective fine inclusions was not sufficient to obtain high HAZ toughness.

These slabs were made into thick plates having a thickness of 50 mm by thick plate rolling, subjected to ultra-high heat input electroslag welding, which is generally used for construction and the like, to obtain welding heat HAZ.
The toughness of each part was evaluated.
While Z toughness was stably exhibited, all steels other than the present invention were low in toughness.

[0079]

Industrial Applicability The present invention relates to a steel material having excellent weld joint toughness even for large heat input welding of 200 kJ / cm ² or more and ultra-high heat input welding, and a method for producing the same. Severe H against destruction of buildings, bridges, etc.
A steel material satisfying the AZ toughness requirement can be supplied. The effect on this kind of industrial field is extremely large, and the contribution to society is very large in terms of structural safety. In addition, the present production method enables stable production of cast slabs in which oxides are finely dispersed, and not only enables production of steel sheets having excellent HAZ toughness, but also causes problems in production of steel sheets caused by coarse inclusions. It is also effective as a solution.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the number of oxide particles in steel and HAZ toughness.

FIG. 2 is a diagram showing a relation between an equivalent equation EN and HAZ toughness.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21C 7/06 C21C 7/06 C22C 38/14 C22C 38/14 38/58 38/58 (72) Inventor Yuji Funatsu 1 Nishinoshu Oita Oita City Nippon Steel Corporation Oita Works (72) Inventor Jun Otani Oita City 1 Nishinoshu Oita City Nippon Steel Corporation Oita Steel Corporation 72 Inventor Tomohiko Hata No. 1, Nishinoshu, Oita City Nippon Steel Corporation Oita Works (72) Inventor Masamitsu Wakao No. 1, Nishinoshu Oita City, Oita City F-term in Oita Works, Nippon Steel Corporation (reference) 4K013 AA07 AA09 BA08 BA14 CE01 CE06 EA18 EA19 EA25 EA30

Claims (10)

[Claims] 1. C: 0.03 to 0.18 by mass%
%, Si: ≤0.5%, Mn: 0.4-2.0%, P
: ≦ 0.02%, S: ≦ 0.02%, Al: 0.0
0.05 to 0.04%, Ti: 0.005 to 0.03%, C
a: 0.0005 to 0.003%, N: 0.0005
-0.006%, O: 0.0005-0.006%, the balance being steel consisting of Fe and unavoidable impurities,
Further, oxide particles having a circle equivalent diameter of 0.005 to 2.0 μm are contained in the steel in a number density per unit area of 100 to 50.
00 / mm ^2, and the composition of the oxide particles is at least Ca, Al, and O elements, and the element excluding O is in mass%, Ca: 5% or more, Al: 5% or more. A steel material with excellent toughness in the heat-affected zone of a weld, characterized in that: 2. C: 0.03 to 0.18 by mass%
%, Si: ≤0.5%, Mn: 0.4-2.0%, P
: ≦ 0.02%, S: ≦ 0.02%, Al: 0.0
0.05 to 0.04%, Ti: 0.005 to 0.03%, C
a: 0.0005% to 0.003%, Mg: 0.0001
-0.002%, N: 0.0005-0.006%,
O: 0.0005 to 0.006%, the balance being F
e) and oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm in a number density per unit area of 100 to 5000 particles / mm ^2. At least Ca, Al,
It is composed of Mg and O elements, and the element excluding O is mass%.
And a steel material having excellent toughness of the weld heat affected zone, characterized in that: Ca: 5% or more, Al: 5% or more, Mg: 1% or more. 3. In mass%, Cu: ≦ 1.0%, Ni: ≦
1.5%, Nb: ≦ 0.03%, V: ≦ 0.1%, C
r: ≦ 0.6%. Mo: ≦ 0.6%, REM: ≦ 0.0
The steel material having excellent toughness of a heat-affected zone of a weld according to claim 1 or 2, which contains 5%. 4. The composition according to claim 1, wherein the composition of the oxide particles further contains 1% or more by mass of S.
3. A steel material excellent in toughness of a weld heat affected zone according to any one of 3. 5. The composition of the oxide particles further comprising Mg
3% or more by mass%.
5. A steel material excellent in toughness of a weld heat affected zone according to any one of the above-mentioned items. 6. The steel according to claim 1, wherein said steel further comprises: B: 0.
0005-0.003%, EN = (% N)-
The weld heat-affected zone toughness according to any one of claims 1 to 5, wherein 0 ≦ EN ≦ 0.002 is satisfied in an equivalent equation of 0.292 (% Ti) -1.292 (% B). Excellent steel material. 7. The oxide particles have a circle equivalent diameter of 0.1 to
The steel material according to any one of claims 1 to 6, wherein the steel material has excellent toughness in a heat-affected zone of a weld. 8. In a secondary refining process in a reduced-pressure atmosphere, Ti, Al, Ca are converted to Ti, A in molten steel having an oxygen concentration of 80 ppm or less.
A molten steel having a chemical composition and an oxide according to any one of claims 1 to 7, which is added in the order of l and Ca to form a cast slab. A method for manufacturing steel with excellent toughness. 9. When the oxygen concentration in the molten steel in the secondary refining step is more than 80 ppm to 200 ppm or less, A
1 or Al alloy is added to reduce the oxygen concentration in the molten steel to 80
9. The method for producing a steel material having excellent toughness in a heat affected zone of a weld according to claim 8, wherein the content is not more than ppm. 10. When the oxygen concentration in the molten steel in the secondary refining step is more than 200 ppm, C is added in a reduced pressure atmosphere and the oxygen concentration in the molten steel is reduced to 200 pp by C deoxidation.
10. The method for producing a steel material having excellent toughness of a weld heat-affected zone according to claim 9, wherein the thickness is not more than m.