JP3378433B2 - Manufacturing method of steel sheet with excellent toughness of weld heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a welding 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. 1
218, "Effect of TiN dispersion state on austenite grain size of low carbon / low alloy steel",
It is generally widely known that fine precipitates stable at high temperature such as TiN are finely dispersed in steel to pin the growth of γ grains. However, among various kinds of carbides and nitrides, T is said to have a pinning effect up to the highest temperature in steel.
Even at iN, 1400 as judged from its solubility product
At high temperatures above ℃, most of the effect is lost due to coarsening and melting of TiN. Therefore, there is no conventional means for suppressing the γ grain growth in a region heated above 1400 ° C. near the melting line of HAZ, and HAZ embrittlement in this region has been a serious problem.

As a means for solving such a problem,
JP-A-60-245768, JP-A-60-152
No. 626, JP-A-63-210235, JP-A-2-250917 and the like disclose intragranular transformation ferrite (IntraGranular Ferr) in coarse γ grains.
It has been attempted to improve HAZ toughness by positively generating (ITE: IGF). In such a case, the grain boundary ferrite (Grain Boundary F
errite: GBF) and coarse ferrite side plate (FS)
Since P) is liable to be generated coarsely, these brittle structures and I
The HAZ toughness is improved as the volume fraction of IGF is increased by competing with the generation of GF. 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. Ba
inite: 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 mechanical properties of the base material. Therefore, it is difficult to select a chemical component that sufficiently satisfies both of them, and there is a limit in increasing the HAZ toughness. Further, it is necessary to produce steel types having different constituent hardenability depending on the welding conditions and the base metal material, which imposes a high manufacturing cost.

Therefore, there has been a strong demand for a new manufacturing method capable of simultaneously achieving cost reduction and high HAZ toughness by increasing the degree of freedom in component design and integrating steel types.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to inexpensively manufacture a thick steel plate having a tensile strength of 500 to 800 MPa and good HAZ toughness under a wide range of welding conditions.

[0007]

Means for Solving the Problems The gist of the present invention is as follows: (1) C: 0.02 to 0.15% by weight% Si: 0.4% or less Mn: 0.05 to 2.0% P: 0.015% or less S: 0.006% or less Al: 0.006% or less Ti: 0.005~0.03% Mg: 0.0015 ~0.004 % N: 0.001~0.005% O : Welding heat characterized by containing 0.001 to 0.0030% of steel, with the balance being iron and unavoidable impurities, into a slab by continuous casting, reheating this to 1250 ° C. or lower, and then performing heat treatment. Affected zone A method for manufacturing a steel sheet with excellent toughness.

(2) C: 0.02 to 0.15% Si: 0.4% or less Mn: 0.05 to 2.0% P: 0.015% or less S: 0.006% or less by weight% Al: 0.006% or less Ti: 0.005 to 0.03% Mg: 0.0015 to 0.004% N: 0.001 to 0.005% O: 0.001 to 0.0030% Furthermore, Cu: 0.5% or less Ni: 0.5% or less Cr: 0.5% or less Mo: 0.5% or less Nb: 0.03% or less V: 0.03% or less Ca: 0.005% REM: 0.005% or less B: 0.0015% or less, one or more of which is contained, and the balance is iron and unavoidable impurities.
A method for producing a steel sheet having excellent toughness in a weld heat affected zone, which comprises reheating to 50 ° C. or lower and then thermomechanical treatment.

The inventors have obtained the following new findings by adding Ti and Mg to steel containing substantially no Al. FIG. 1 shows the influence of the amount of Mg on the oxide fractional state. As the amount of Mg increases, the number of oxides increases and the particle size decreases. Fine dispersion of such oxides is low A
It was found for the first time that it was expressed only in case of 1. The reason why the oxide is finely dispersed is considered to be that the oxide in the molten steel becomes finer due to the strong acid action peculiar to Mg, and the fine oxide generated during solidification increases. 2 is 145
The influence of the amount of Mg on the γ grain size heated at 0 ° C. is shown. The γ-grains become finer as the amount of Mg increases. This is because the finely dispersed oxides exist stably at 1450 ° C. and pin the γ grain growth. FIG. 3 shows the influence of the number of oxides on the γ grain boundaries on the numbers and grain sizes of GBF 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 oxides on the γ grain boundary are GBF and PS.
This is because it acts as a P nucleation site. FIG. 4 shows the effect of the amount of Mg on the oxide particle size distribution. As the amount of Mg increases, the number of coarse oxides decreases. 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.

That is, the technical idea of the present invention is to finely disperse oxides by adding Ti and Mg in combination to low Al steel, to refine heating γ in the HAZ near the melting line and to reduce GBF.
And by miniaturizing the FSP, the HAZ structure is miniaturized,
Further, it is to reduce coarse oxides and improve 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 C is the minimum amount for securing the strength and toughness of the base material and the welded portion. However, if the amount of C is too large, the toughness of the base material and HAZ is reduced and the weldability is deteriorated, so the upper limit was made 0.15%.

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 0.05%.
However, although Mn is preferably 0.5% or more, too much Mn deteriorates HAZ toughness, promotes center segregation of the slab, and deteriorates weldability, so the upper limit was made 2.0%.

In the steel of the present invention, P and S which are impurity elements
Of 0.015% or less and 0.006% or less, respectively, because the slab center deviation is reduced and the base metal and H
This is to improve the mechanical properties of AZ. Reduction of P is H
The grain boundary destruction of AZ 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%, respectively.
It is the following.

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, A1 is an impurity element and the smaller the better.

Ti is an essential element of the present invention, and is required to be 0.005% or more in order to form Ti-based oxide and TiN which are effective in refining the HAZ structure. In the present invention, in order to further heat γ-fine particles 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 precipitation of excess TiC, and was set to 0.03%.

Mg is the most important element of the present invention, and when it is added to Ti to low A1 steel in a complex manner, the oxide is finely dispersed, and HAZ near the melting line heated above 1400 ° C.
Also in, a fine structure can be obtained. The lower limit is the minimum amount at which this effect is exhibited, and the upper limit is the amount at which this effect is saturated. Addition of Mg in excess of the upper limit is not preferable because it increases alloy cost.

N is an essential element for forming TiN and improving the HAZ toughness. The lower limit is a sufficient amount of TiN
Is the minimum amount for ensuring the
It is an amount for preventing AZ arm formation. In the present invention, TiN
Make the most of the pinning effect of.

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. Therefore, it is set to 0.001 to 0.0030% .

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

Cu and Ni improve the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness.
Each upper limit of 0.5% or less of each element is weldability and H
This is to prevent deterioration of the AZ toughness.

Mo improves the strength and toughness of the base material. However, if the addition amount of Al 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 initiality 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 0.03%, the HAZ toughness deteriorates.

V improves the strength of the base material, but 0.03%
If it exceeds, the weldability and HAZ toughness will be impaired.

The reason why Ca and REM are added is to control the morphology of the stretched inclusions (MnS) and improve the toughness. However, if the addition amount of these exceeds 0.005%, a large amount of coarse oxide is formed, and the base metal and HAZ
Deteriorates toughness.

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-quenching-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.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION (Examples) Table 1 shows the chemical composition of continuously cast steel, Table 2 shows steel plate manufacturing conditions and base material, and Table 3 shows HAZ toughness. Steel sheets were welded by various welding methods and various welding heat inputs, and the Charpy impact characteristics of HAZ 1 mm and the fusion line (FL), which is the most armed portion of HAZ, were investigated. The steel of the present invention is TS 650 -82
It has a good base material having a vTrs of -80 ° C or lower at 0 MPa, and has a good HAZ toughness in the HAZ near the fusion line 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 chemical composition and slab heating conditions are not appropriate. Steel 6 is A
Since the amount of 1 is too large, the oxide fine dispersion effect due to addition of Mg is not exhibited, and the HAZ heated γ grains become coarse, resulting in poor HAZ toughness. Steel 7 contains too little Ti, so Ti-based oxides and T
Insufficiently generated iN, HAZ heated γ grains become coarse and HA
Z toughness is inferior. Steel 8 has too much Ti, so 1400 ° C
In the HAZ heated above, the solid-soluted Ti excessively precipitates as TiC during the cooling process and embrittles the HAZ. Since Steel 9 has a small amount of Mg, the effect of fine dispersion of the dispersed substance is insufficient and the HAZ heated γ grains are coarsened to deteriorate the HAZ toughness. Steel 10
However, since the amount of N is too small, the formation of TiN is insufficient, the HAZ heated γ grains become coarse, and the HAZ toughness deteriorates. Since the steel 11 contains too much N, the HAZ toughness deteriorates due to an increase in solute N. Steel 12 has too little O, so that a sufficient amount of oxide is not formed and HAZ heated γ grains are coarsened, resulting in poor HAZ toughness. Steel 13 has an excessive amount of O, so that the cleanliness of the steel is lowered and the coarse oxide that becomes the starting point of fracture increases, resulting in poor HAZ toughness. Steel 14 has a too high slab heating temperature, so Ti
N becomes coarse and HAZ heating γ grains become coarse and HA
Z toughness is inferior.

Although the present invention is preferably applied to a thick plate mill, it can also be applied to hot coils, shaped steel and the like.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

According to the present invention, it is possible to integrate steel types by component design with priority given to the base material, and it has become possible to inexpensively produce high strength steel excellent in HAZ toughness. In addition, since a good HAZ toughness is achieved under a wide range of welding conditions, the safety of the welded structure is significantly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the amount of Mg on the oxide dispersion 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 (72) Inventor Atsuhiko Yoshie 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works (56) Reference JP-A-5-43977 (JP, A) JP-A-5- 171341 (JP, A) JP 7-278738 (JP, A) JP 9-227985 (JP, A) JP 9-176730 (JP, A) JP 9-279234 (JP, A) Patent 3256118 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/46-9/48 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.02 to 0.15% Si: 0.4% or less Mn: 0.05 to 2.0% P: 0.015% or less S: 0.006% or less Al by weight% 0.006% or less Ti: 0.005~0.03% Mg: 0.0015 ~0.004 % N: 0.001~0.005% O: containing 0.001 to 0.0030 percent, A method for producing a steel sheet having excellent toughness in a welded heat-affected zone, which is characterized in that steel having a balance of iron and unavoidable impurities is continuously cast to form a slab, and the slab is reheated to 1250 ° C. or lower and then heat-treated. 2. C: 0.02 to 0.15% Si: 0.4% or less Mn: 0.05 to 2.0% P: 0.015% or less S: 0.006% or less Al by weight 0.006% or less Ti: 0.005~0.03% Mg: 0.0015 ~0.004 % N: 0.001~0.005% O: containing 0.001 to 0.0030 percent, Furthermore, Cu: 0.5% or less Ni: 0.5% or less Cr: 0.5% or less Mo: 0.5% or less Nb: 0.03% or less V: 0.03% or less Ca: 0.005% or less REM: 0.005% or less B: 0.0015% or less, one or more of which is contained, and the balance is iron and unavoidable impurities.
A method for producing a steel sheet having excellent toughness in a weld heat affected zone, which comprises reheating to 50 ° C. or lower and then thermomechanical treatment.