JPH09310119A - Welding heat-affected zone steel plate manufacturing method with excellent toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thick steel plate excellent in toughness in the heat affected zone(HAZ) and strength, at the time of subjecting a steel to continuous casting, by reheating a slab obtd. by adding an Ni-Mg alloy to a low Al molten steel and executing componental regulation and thereafter subjecting the same to thermomechanical treatment. SOLUTION: At the time of refining a steel and executing continuous casting, an Ni-Mg alloy contg. <=50wt.% Mg is added to a molten steel. In this way, a slab contg. 0.02 to 0.20% C, <=0.4% Si, 0.5 to 2.0% Mn, <=0.015% P, <=0.006% S, <=0.006% Al 0.005 to 0.03% Ti, 0.0005 to 0.005% Mg, <=1.5% Ni, 0.001 to 0.005% N, 0.002 to 0.006% O, and the balance Fe with inevitable impurities is obtd. This slab is reheated at <=1250 deg.C to regulate the constituting phases and grain size of the base metal. Next, thermomechanical treatment such as controlled rolling, controlled rolling-accelerated cooling, controlled rolling- quenching-tempering is executed.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a heat affected zone (He).
at Affected Zone (HAZ) is a method for producing a thick steel sheet having excellent toughness and is applied in the steel industry. The steel sheet produced by the present invention is used for various welded structures such as construction, bridges, shipbuilding, line pipes, construction machines, marine structures, and tanks.

[0002]

2. Description of the Related Art In the heat affected zone (HAZ), the heating temperature during welding becomes higher as it gets closer to the melting line, and especially in the region near 1100 ° C. near the melting line where the heating austenite (γ) is extremely coarse. Therefore, the HAZ structure after cooling becomes coarse and the toughness deteriorates.

As a method of refining the heated γ grains of steel,
"Iron and Steel," No. 62 (1976) No. 9, p. 1209-
p. As described in 1218 “Effect of TiN dispersion state on austenite grain size of low carbon / low alloy steel”, high-humidity and stable precipitates such as TiN are finely dispersed in the steel to grow γ grains. Pinning is generally well known. However, even for TiN, which is said to have the pinning effect up to the highest temperature in steel among various carbides / nitrides, as is judged from its solubility product, 1
At a high temperature of 400 ° C. or higher, most of the effect is lost due to coarsening and melting of TiN.

Therefore, as in the vicinity of the HAZ fusion line, 14
There is no conventional means for suppressing γ grain growth in the region heated above 00 ° C., and HAZ embrittlement in this region has been a serious problem. As means for solving such problems, Japanese Patent Laid-Open Nos. 60-245768 and 60-152626 are available.
JP-A-63-210235, JP-A-2-
No. 250917 discloses an intragranular transformation ferrite (IntraGranular Ferrit) in a coarse γ grain.
The HAZ toughness has been improved by positively generating (e: IGF). In such a case, the grain boundary ferrite (Grain Boundary Fer)
(Rite: GBF) and coarse ferrite side plates (FSP) are likely to be generated in a coarse manner, so that the formation of these embrittlement structures and IGF competes with each other, and the larger the volume fraction of IGF, the more H
The AZ toughness is improved. In order to suppress the formation of coarse GBF and FSP, it is necessary to enhance the hardenability of the γ grain boundary, but if the hardenability is excessively increased, the coarse upper bainite (Upper) containing island martensite (Upper) is required. Baini
te: Bu) is generated and the IGF fraction is lowered. Therefore, from the viewpoint of HAZ toughness, it is important to secure appropriate hardenability corresponding to welding conditions (cooling rate). On the other hand, hardenability must also be considered from the viewpoint of the base material. However, it is difficult to select a chemical composition that satisfies both requirements, and the HAZ toughness is limited.

[0005]

The present invention has a tensile strength of 40 with good HAZ toughness under a wide range of welding conditions.
An object is to manufacture a thick steel plate having a pressure of 0 MPa or more.

[0006]

The gist of the present invention is to add a Ni--Mg alloy having a Mg content of 50% by weight or less to molten steel when refining and continuously casting the steel.
C: 0.02 to 0.20%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.015%
Hereinafter, S: 0.006% or less, Al: 0.006
% Or less, Ti: 0.005 to 0.03%, Mg:
0.0005 to 0.005%, Ni: l. 5% or less,
N: 0.001-0.005%, O: 0.00
2 to 0.006%, and if necessary, C
u: 1.5% or less, Cr: 0.5% or less, Mo:
0.5% or less, Nb: 0.05% or less, V: 0.
05% or less, Ca: 0.005% or less, REM: 0.
A steel slab containing at least one of 005% or less and B: 0.0015% or less and the balance consisting of iron and unavoidable impurities is produced, which is reheated to 1250 ° C. or less and then thermomechanically treated. .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors have obtained the following new knowledge by adding Ti and Mg to steel containing substantially no Al. FIG. 1 shows the influence of the amount of Mg on the oxide dispersion state. As the amount of Mg increases, the number of oxides increases and the particle size decreases. It has been found for the first time that such fine dispersion of oxides is exhibited only in the case of low Al. It is considered that the oxide is finely dispersed because the oxide in the molten steel becomes fine due to the strong deoxidizing action peculiar to Mg, and the fine oxide generated during solidification increases. FIG.
FIG. 4 is a graph showing the influence of the amount of Mg on the 1450 ° C. heated γ particle size. The γ-grains become finer as the amount of Mg increases. This is because the finely dispersed oxide is stable at 1450 ° C.
This is because the γ grain growth is pinned. Figure 3 is GB
It is a figure which shows the influence of the number of oxides on a (gamma) grain boundary which affects the number and particle size of F and FSP. GBF and FSP are miniaturized due to an increase in the number of oxides on the γ grain boundaries. This is because the oxide on the γ grain boundary acts as a nucleation site for GBF and FSP. FIG. 4 shows the effect of the amount of Mg on the oxide particle size distribution. The increase in the amount of Mg reduces the number of coarse oxides. A larger oxide is more likely to act as a starting point of fracture, and an increase in the number of such oxides embrittles steel. Here, Mg is an element having a very high vapor pressure, and it is a subject of practical application that the yield is extremely small even if it is added to molten steel.

As a technique for improving the Mg yield, for example, Japanese Patent Laid-Open No. 7-48616 discusses the types of additive alloys, such as Si--Mg alloy, Fe--Mn--Mg alloy, and Al.
-Mg alloy is effective in improving the Mg yield, but
Since the steel of the present invention contains substantially no Al, an Al-Mg alloy cannot be used. Therefore, as a result of studying an additive alloy in which Mg is most efficiently retained within the range of the chemical composition of the present invention, it was found that a Ni-Mg alloy is very effective as shown in Table 1. The reason is that Mg in the alloy surface layer evaporates at the moment when the alloy is added to the molten steel,
It is considered that the surface layer becomes Ni-rich in the molten state and the Ni-rich layer suppresses evaporation of Mg inside the alloy.
As a place to add the alloy, a ladle ladle, a tundish or a mold of continuous casting can be considered, but it is preferable to add it in a continuous casting mold because the shorter the time from alloy addition to solidification is to the yield. . At this time, it is effective to improve the Mg yield by filling the iron wire with the alloy and continuously adding it to the molten steel.

As described above, the technical idea of the present invention is that Ni-Mg
An oxide is used to finely disperse oxides by adding Ti and Mg together to a low-Al steel using an alloy, and the heating γ in the HAZ near the melting line is refined and the GBF and FSP are refined to produce H.
AZ structure is made finer and coarser oxides are reduced,
It is to improve the HAZ toughness. Since this effect is exhibited in a wide range of welding conditions, it is possible to design the component with priority given to the base metal material. Therefore, the present invention simultaneously achieves the reduction of manufacturing cost by the integration of steel types and the good HAZ toughness.

The reasons for limiting the chemical components will be described below.

The lower limit of 0.02% of C is the minimum amount for securing the strength and toughness of the base material and the welded portion. But C
If the content is too large, the toughness of the base material and the HAZ is reduced and the weldability is deteriorated. Therefore, the upper limit is set to 0.20%.

Si is contained in steel for deoxidation, but if it is too much, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.4%. Deoxidation of steel is sufficiently possible with Ti alone, and in order to obtain good HAZ toughness, Si is preferably 0.3% or less.

Since Mn is indispensable for securing the strength and toughness of the base material and the welded portion, the lower limit was made 0.5%. However, if the Mn content is too large, the HAZ toughness is deteriorated, the center of the slab is promoted, and the weldability is deteriorated. Therefore, the upper limit is set to 2.0%.

Impurity elements P and S in the steel of the present invention
Of 0.15% or less and 0.006% or less, respectively, because the slab center deviation is reduced and the base metal and HA
This is to improve the mechanical properties of Z. Reduction of P is HA
The grain boundary destruction of Z is suppressed, and the reduction of S improves the materials of the base material and HAZ in the plate thickness direction through the reduction of MnS. Preferred P and S are 0.01% or less and 0.003% or less, respectively.

Al is an unfavorable element in the present invention and is set to 0.006% or less. This is 0.006% Al
This is because the effect of Mg, which is the essence of the present invention, will not be exhibited if the content is exceeded. Al is usually used as a deoxidizing element, but deoxidation is possible with Ti alone. In the present invention, Al is an impurity element, and the smaller the better, the better.

Ti is an essential element of the present invention, and is required to be 0.005% or more in order to form a Ti-based oxide and TiN which are effective in refining the HAZ structure. In the present invention, in order to achieve further heating γ-fine graining in the low temperature heating region, TiN is utilized to the maximum in addition to the oxide, and a strong pinning effect is exhibited at 1350 ° C. or lower. The upper limit of Ti is to prevent HAZ embrittlement due to excessive TiC protrusion,
It was set to 0.03%.

Mg is the most important element of the present invention, and M
Oxides are finely dispersed by complex addition of Ti to low Al steel using a Ni-Mg alloy having a g content of 50% by weight or less, and H near the melting line heated above 1400 ° C.
A fine structure can also be obtained in AZ. If the Mg content of the Ni-Mg alloy exceeds 50% by weight, the reaction with molten steel is severe during the addition, and it is difficult to use in an actual production line. Further, the lower limit of 0.0005% of the amount of Mg in steel is the minimum amount at which the HAZ microscopic effect is exhibited, and the upper limit of 0.005%.
Is the amount by which this effect saturates. The addition of Mg in excess of the upper limit is not preferable because it only increases the alloy cost.

Ni is necessarily included because of the use of Ni-Mg alloys. However, if the Ni content exceeds 1.5%, the weldability and HAZ toughness deteriorate, so the upper limit is 1.5.
%. N is an essential element for forming TiN and improving HAZ toughness. The lower limit is the minimum amount to secure a sufficient amount of TiN, and the upper limit is HA due to solute N.
It is an amount for preventing Z embrittlement. The present invention makes full use of the pinning effect of TiN.

O is essential for forming a fine oxide by combining with Mg and Ti. The lower limit is the minimum amount for ensuring a sufficient amount of oxide, and the upper limit is the maximum amount for ensuring the cleanliness of steel and avoiding deterioration of mechanical properties.

Next, Cu, Ni, Mo, Cr, Nb,
The reason for adding at least one of V, Ca, REM, and B will be described.

Cu improves the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness. The upper limit of 1.5% is the maximum amount for preventing deterioration of weldability and HAZ toughness.

Mo improves the strength and toughness of the base material. However, if the addition amount exceeds 0.5%, the base material toughness, weldability and HAZ toughness are impaired.

Cr improves the strength of the base material. However, if the addition amount exceeds 0.5%, the base material toughness, weldability and HAZ toughness are impaired.

Nb is an element effective for refining the base metal structure, and improves the strength and toughness of steel. However, if the addition amount exceeds 0.05%, the HAZ toughness deteriorates.

V improves the strength of the base metal, but it is 0.05%.
If it exceeds, weldability and HAZ toughness are impaired.

The reason for adding Ca and REM is to control the morphology of the stretched inclusions (MnS) and improve the toughness. However, if the amount of addition exceeds 0.0050%, a large amount of coarse oxides is formed, and the base material and HA
It degrades the toughness of Z.

B improves the hardenability and improves the strength and toughness of the base material and HAZ. However, if added in excess of 0.0015%, HAZ toughness and weldability deteriorate.

Even if the steel components are limited as described above, if the production method is not appropriate, fine oxides and TiN are added to the steel before welding.
Cannot be dispersed. Therefore, it is necessary to limit the manufacturing conditions.

It is essential that steel is industrially produced by a continuous casting method. This is because the continuous casting method has a high solidification rate, and thus a large amount of fine oxides and TiN can be obtained in the slab. At this time, the cooling rate varies depending on the slab thickness, and a slab thickness of 350 mm or less is desirable from the viewpoint of HAZ toughness. Further, the reheating temperature of the slab needs to be 1250 ° C or lower. When heated to a temperature higher than 1250 ° C, TiN becomes coarse and HAZ heating γ
Ineffective in suppressing grain coarsening. It is not always necessary to reheat the slab, and there is no problem even if hot charge rolling or direct rolling is performed. Thermomechanical treatment is essential for the rolling method. This is an excellent HA
This is because even if Z toughness is obtained, it is insufficient as a steel material if the mechanical properties of the base material are inferior. It is necessary to control the constituent phase and crystal grain size of the base material by thermomechanical treatment to achieve the desired strength and toughness. Methods of thermo-mechanical treatment include 1) controlled rolling, 2) controlled rolling-accelerated cooling, 3) controlled rolling-burning-tempering. It should be noted that even if this steel is reheated to a temperature of Ac ₁ or lower for the purpose of dehydrogenation after production, the characteristics of the present invention are not impaired.

[0030]

EXAMPLES Table 2 shows the chemical composition of continuously cast steel, Table 3 shows steel plate manufacturing conditions and base material, and Table 4 shows HAZ toughness.
Steel sheets were welded under various welding conditions, and the fusion line (FL), which is the most embrittled portion of HAZ, and the Charpy impact property of HAZ 1 mm were investigated. The steel of the present invention has a good base material having a TS of 450 to 820 MPa and a VTrs of −80 ° C. or lower,
It has good HAZ toughness in the vicinity of FL where the welding heat input is 30 to 1000 kJ / cm. On the other hand, the comparative steel cannot obtain good HAZ toughness because the added Mg alloy, chemical composition, and slab heating conditions are not appropriate. Since Steel 6 contains too much Al, the oxide fine dispersion effect due to the addition of Mg is not exhibited and the HAZ heated γ grains become coarse, resulting in poor HAZ toughness. Steel 7
Since the amount of Ti is too small, Ti-based oxides and TiN are not sufficiently generated, and the HAZ heated γ grains become coarse, resulting in poor HAZ toughness. Steel 8 has too much Ti, so it is heated above 1400 ℃.
Excessively precipitates to embrittle the HAZ. Steel 9 is Fe-
Since Mg was added using a Si-10 wt% Mg alloy, the amount of Mg in the steel was small, and the fine oxide dispersion was insufficient and HAZ
The heated γ grains become coarse and the HAZ toughness is poor. In Steel 10, since the amount of N is too small, the formation of TiN is insufficient, and the HAZ heated γ grains become coarse, resulting in poor HAZ toughness. Since the steel 11 contains too much N, the HAZ toughness deteriorates due to an increase in solute N. Steel 1
In the case of No. 2, the amount of O was too small, so that a sufficient amount of oxide was not generated and the HAZ heated γ grains were coarsened, resulting in poor HAZ toughness. Steel 13
Since the amount of O is too large, the cleanliness of the steel is lowered and the coarse oxides that become the starting point of fracture increase, resulting in poor HAZ toughness. Steel 1
In No. 4, since the slab heating temperature is too high, TiN is coarsened, and HAZ heated γ grains are coarsened, resulting in poor HAZ toughness.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

According to the present invention, good HAZ toughness is achieved under a wide range of welding conditions and base metal materials, and the safety of various welded structures is significantly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the amount of Mg on the oxide fraction state.

FIG. 2 is a diagram showing the influence of the amount of Mg on the γ particle size heated at 1450 ° C.

FIG. 3 is a diagram showing the effect of the number of oxides on a γ grain boundary on the number and grain size of GBF and FSP.

FIG. 4 is a diagram showing the influence of the amount of Mg on the oxide particle size distribution.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/14 C22C 38/14

Claims (2)

[Claims] 1. When refining and continuously casting steel, a Ni-Mg alloy having an Mg content of 50% by weight or less is added to molten steel so that C: 0.02 to 0. 20%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.015% or less, S: 0.006% or less, Al: 0.006% or less, Ti: 0. 005-0.03%, Mg: 0.0005-0.005%, Ni: l. 5% or less, N: 0.001 to 0.005%, O: 0.002 to 0.006%, and a balance made of iron and inevitable impurities was made into a steel slab, which was reheated to 1250 ° C or less. A method for producing a steel sheet having excellent toughness in a weld heat affected zone, characterized by performing thermomechanical treatment after heating. 2. When refining and continuously casting steel, a Ni-Mg alloy having a Mg content of 50% by weight or less is added to the molten steel so that C: 0.02 to 0. 20%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.015% or less, S: 0.006% or less, Al: 0.006% or less, Ti: 0. 005-0.03%, Mg: 0.0005-0.005%, Ni: 1.5% or less, N: 0.001-0.005%, O: 0.002-0.006%, In addition, Cu: 1.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.005% or less. , REM: 0.005% or less, B: 0.0015% or less, containing at least one of the following, with the balance being iron and unavoidable A method for producing a steel sheet having excellent toughness in a weld heat affected zone, which comprises producing a steel slab made of a pure material, reheating the steel slab to 1250 ° C. or lower, and then subjecting it to thermomechanical treatment.