JP3426047B2 - Method for producing low yield ratio 590 N / mm2 class high strength steel with excellent weldability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low yield ratio 590 N / mm ² class high strength steel having excellent weldability. In the steel industry, it is most preferable to apply it to a thick plate mill. It can also be applied to hot coils, shaped steel, etc.

[0002]

2. Description of the Related Art Conventionally, a thick (plate thickness of 50 mm or more) 59
0N / mm ² grade high strength steel (hereinafter referred to as HT60) is B
Steel with high hardenability by addition or large addition of various alloys has been manufactured by quenching and tempering. However, as described above, with the addition of B and the addition of a large amount of alloy, the weldability is remarkably poor, and preheating at 100 ° C. or higher is required from the viewpoint of welding crack prevention at the time of on-site welding work, resulting in a significant decrease in work efficiency and work environment. It was getting worse.

On the other hand, steel utilizing precipitation hardening is known as HT60 having good weldability. However, in high-rise building steel, a steel with a low yield ratio (YR) (YR ≦ 80%) is used to absorb the energy during an earthquake and prevent the building from collapsing.
However, the steel using precipitation hardening had a high yield ratio and poor seismic resistance.

[0004]

SUMMARY OF THE INVENTION The present invention is an HT for construction.
The purpose is to provide 60 inexpensive manufacturing techniques. The HT60 manufactured based on the present invention is less likely to cause welding cracks even in small heat input welding and constraint welding,
Preheating can be reduced or omitted in welding work, and the building using this HT60 absorbs the energy of the earthquake and has excellent earthquake resistance.
With features such as.

[0005]

The present invention solves the above-mentioned problems and achieves the objects, and the gist thereof is as follows. (1) C: 0.04 to 0.11% by weight ratio, Si:
0.5% or less, Mn: 0.9 to 1.6%, P: 0.03
% Or less, S: 0.01% or less, V: 0.10 to 0.15
%, Ti: 0.005-0.025%, Al: 0.1%
Below, N: 0.006% or less and Di≡0.367√
C (1 + 0.7Si) (1 + 3.33Mn) (1 + 0.
35Cu) (1 + 0.36Ni) (1 + 2.16Cr)
(1 + 3Mo) (1 + 1.7V) (1 + 1.77Al)
= 1.5 to 3.5, Cr: 0.05 to 0.50
%, Mo: 0.05 to 0.30%, Ni: 0.05 to
Steel containing at least 2.0%, Cu: 0.05 to 1.5%, or at least two, and substantially free of B containing the balance Fe and unavoidable impurities, is hot-rolled to Ac. ₃ to 1
Reheat to a temperature range of 000 ° C and quench, then 750
A plate having a thickness of 5 after being heated again to 850 ° C., quenched, and subsequently tempered in a temperature range of Ac ₁ transformation point or lower.
Low yield ratio 590 N / mm ² with excellent weldability of 0 mm or more
Of high grade high strength steel.

(2) C: 0.04 to 0.11 by weight ratio
%, Si: 0.5% or less, Mn: 0.9 to 1.6%,
P: 0.03% or less, S: 0.01% or less, V: 0.1
0 to 0.15%, Ti: 0.005 to 0.025%, A
l: 0.1% or less, N: 0.006% or less, and Di≡
0.367√C (1 + 0.7Si) (1 + 3.33M
n) (1 + 0.35Cu) (1 + 0.36Ni) (1+
2.16Cr) (1 + 3Mo) (1 + 1.7V) (1+
In the range of 1.77Al) = 1.5 to 3.5, Cr: 0.
05-0.50%, Mo: 0.05-0.30%, N
i: 0.05 to 2.0%, Cu: 0.05 to 1.5%, one or more, and Nb: 0.01 to 0.05
%, Ca: 0.001 to 0.006% of one or two, and the balance of Fe and unavoidable impurities and substantially B-free steel, after hot rolling, Ac _{3 to} 100
Reheat to 0 ° C temperature range and quench, then 750-8
A plate having a thickness of 50 m, which is characterized by being heated again to 50 ° C., quenched, and subsequently tempered in a temperature range below the Ac ₁ transformation point.
A method for producing a high yield steel having a low yield ratio of 590 N / mm ^{2 and} excellent in weldability of m or more.

[0007]

The present invention will be described below. According to the research conducted by the present inventor, in order to simultaneously realize excellent weldability and low yield ratio of thick (sheet thickness 50 mm or more) HT60, B addition is not added, precipitation hardening, and appropriate steel components are fired. It was found that it is necessary to adjust the range so that the acceptability is obtained. Further, it was found that the steel in which the crystal grains were refined by adding a small amount of Ti and optimizing the heat treatment conditions exhibited excellent low temperature toughness even when precipitation hardening was performed.

The minimum amount of V required to obtain the characteristics of the HT60 is 0.10%. However, V amount is 0.1
If it exceeds 5%, it becomes difficult to sufficiently reduce YR, so 0.15% was made the upper limit. Further, in order to reduce YR to a predetermined value (80%) or less, it is necessary to adjust the hardenability of steel to an appropriate range, and Di≡0.367√C
(1 + 0.7Si) (1 + 3.33Mn) (1 + 0.3
5Cu) (1 + 0.36Ni) (1 + 2.16Cr)
(1 + 3Mo) (1 + 1.7V) (1 + 1.77Al)
It is necessary to adjust within the range of 1.5 to 3.5 with the index indicated by. The reason is that if it is less than 1.5, YR is not sufficiently reduced by the two-phase structured heat treatment described later, and if it exceeds 3.5, the standard strength as HT60 is exceeded, and further the weldability deteriorates. This is because

Next, in order to sufficiently exert the above-mentioned precipitation effect and sufficiently lower YR, the manufacturing method must be appropriate. Therefore, it is necessary to limit the heat treatment conditions after hot rolling. First, in order to obtain the strength and excellent low temperature toughness of HT60, it is necessary to refine the structure by solid solution of V and retransformation into γ by reheating after hot rolling. The lower limit of is the Ac ₃ transformation point. If the reheating temperature is too high, the crystal grains become large and the low temperature toughness deteriorates, so the upper limit must be set to 1000 ° C.

Then, after reheating to 750 to 850 ° C., quenching is performed again, and reheating is performed to a temperature of Ac ₁ or lower to perform tempering treatment.
The reason for reheating and quenching at 750 to 850 ° C. is to reduce the yield ratio. In general, the steel precipitation hardened with V has a significantly high YR. Therefore, (γ + α) 2 at 750 to 850 ° C.
Reheat and quench the phase region. By partially γ-transforming, the untransformed region is softened, the γ-transformed region is hardened, and the microstructure is made into a two-phase (soft phase and hard phase), and the yield ratio can be reduced. If the reheating temperature is less than 750 ° C,
The above-mentioned effect cannot be obtained because the region transformed to γ is small. However, when the reheating temperature exceeds 850 ° C., most of the γ-transformation does not occur and the desired two-phase structure cannot be obtained, so that low YR cannot be achieved.

The tempering treatment is necessary to exert the precipitation effect.
It's a ground. However, the temperature is Ac ₁Strength above the point
Is significantly reduced, Ac₁Must be below the point
No (desirable tempering temperature is 500-650 ° C).
However, even if the amount of V added and the manufacturing method are appropriate,
If the components are not suitable, the excellent properties as HT60 will be obtained.
I can't. Hereinafter, this point will be described.

The lower limit of 0.04% of C is the minimum amount for ensuring the strength of the base material and the weld and for exerting these effects when Nb and V are added. However, if the amount of C is too large, the weldability is significantly deteriorated, so the upper limit is 0.11%.
And If a large amount of Si is added, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.5%. Deoxidation of steel is A
Only 1 and Ti are sufficient, and Si is not necessarily added.

Mn is an element essential for securing strength and toughness, and its lower limit is 0.9%. However, Mn
If the amount is too large, the hardenability increases and the weldability and HAZ toughness deteriorate, so the upper limit was made 1.6%. The reason why the impurities P and S in the steel of the present invention are 0.03% or less and 0.01% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S content prevents deterioration of toughness due to MnS. The preferable amounts of P and S are 0.01% or less and 0.005% or less, respectively.

[0014] Cr is an element that enhances the strength of the base material and the welded portion, and at least 0.05% is necessary. However, if too large, the weldability and HAZ toughness are significantly deteriorated, so the upper limit was made 0.50%. Mo is an element that improves both strength and toughness, and 0.05% or more is essential for HT60. However, if it is too large, it is not preferable in terms of weldability and HAZ toughness, and the upper limit is 0.30%.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness.
If it is less than 5%, the effect is weak, and if it exceeds 2.0%, the cost is extremely high and the economy is lost, so the content was made 0.05 to 2.0%. Cu has an effect similar to that of Ni, and also has an effect of increasing strength due to Cu precipitates, improving corrosion resistance, and improving weather resistance. In this case, if the Cu content exceeds 1.5%, the precipitation effect is saturated, and if it is less than 0.05%, there is no effect, so the Cu content is limited to 0.05 to 1.5%.

Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, Ti oxide is formed to improve the HAZ toughness, but if it is less than 0.005%, it has no effect, and if it exceeds 0.025%, it has an unfavorable effect on the HAZ toughness. 0.005-0.025%
Limited to Al is an element generally contained in deoxidized upper steel, but since Si and Ti also perform deoxidation,
In the present invention, the lower limit is not limited. However, if the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the weld deteriorates, so the upper limit was made 0.1%.

Although N is generally contained in steel as an unavoidable impurity, it combines with Nb and V to form a carbonitride to increase the strength, and forms TiN to form the above-mentioned N. To enhance the properties of HT60. Therefore, the N content must be at least 0.001%. However, if the amount of N increases, the HAZ toughness deteriorates and the surface defects of the continuous cast slab are generated, so the upper limit is 0.006%.
And

The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved by this. However, in order to further improve the characteristics for the object, the elements described below, that is, Nb and Ca are selectively selected. By adding,
More favorable results are obtained with respect to improvement in toughness.
Nb forms fine carbonitrides to increase strength and improve HAZ toughness. However, the Nb content is 0.01%
If less than 0.05%, the effect is small, and if over 0.05%, YR
Since it becomes difficult to sufficiently reduce the amount of Nb, the amount of Nb added is set to 0.01 to 0.05%.

Ca controls the morphology of sulfide (MnS),
It has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the Ca content is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, C
Since a large amount of aO and CaS are formed and become large inclusions, not only the toughness of the steel but also the cleanliness is impaired, and the weldability and lamella tear resistance are adversely affected, so the range of Ca addition is 0.0
It was set to 01 to 0.006%.

[0020]

EXAMPLE A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and its strength, toughness, weldability (y-slit cracking property) and the like were investigated. Table 1 and Table 2 (continued from Table 1)
1 to 6 show the chemical composition of the present invention steel, and 7 to 14 show the chemical composition of the comparative steel. Further, Table 3 shows the steel sheet manufacturing conditions of the present invention steel and the comparative steel, and their mechanical properties and weldability.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

As is clear from Tables 1 to 3, in Examples 1 to 6 of the present invention, the strength, YR, toughness and weldability of the base material were achieved in a well-balanced manner. On the other hand, Comparative Steel 7 has a low Di value and thus has a high YR. Comparative steel 8
Has a high C content, and the y slit crack stop temperature is very high. In Comparative Steel 9, since B is added, the y slit crack stop temperature is extremely high. Comparative steel 1
At 0, the amount of V is low and the strength is insufficient. Further, in Comparative Steel 11, since the Di value is too high, the strength becomes extremely high, and exceeds the standard strength of HT60. Further, in Comparative Steel 12, the reheating temperature during quenching in the two-phase region is as low as 730 ° C., the γ conversion is insufficient, and the YR is high. Comparative steel 13
However, since the initial quenching temperature is as high as 1030 ° C. and the crystal grains cannot be sufficiently refined, the base material toughness is poor. In Comparative Steel 14, the reheating temperature during quenching in the two-phase region is as high as 860 ° C., and most of it is γ-ized, so that YR is high.

[0025]

INDUSTRIAL APPLICABILITY As described above, steel materials such as thick steel plates, shaped steels and hot coils manufactured according to the present invention have a low yield ratio HT60 excellent in weldability. As a result, the welding work efficiency and safety at the site are significantly improved, and the safety of buildings and the like can be greatly enhanced.

Claims (2)

(57) [Claims] 1. By weight ratio, C: 0.04 to 0.11%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0. 0.01% or less, V: 0.10 to 0.15%, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, and Di≡0.367√ C (1 + 0.7Si) (1 + 3.3
3Mn) (1 + 0.35Cu) (1 + 0.36Ni)
(1 + 2.16Cr) (1 + 3Mo) (1 + 1.7V)
In the range of (1 + 1.77Al) = 1.5 to 3.5, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.30%, Ni: 0.05 to 2.0%, Cu: A steel containing at least one of 0.05 to 1.5% and substantially no B containing the balance Fe and unavoidable impurities, after hot rolling, at a temperature of Ac _{3 to} 1000 ° C. Weldability with a plate thickness of 50 mm or more, characterized by being reheated to a temperature range, quenched, further heated to 750 to 850 ° C., quenched, and subsequently tempered in a temperature range of Ac ₁ transformation point or lower. Of excellent high yield ratio 590 N / mm ² class high strength steel. 2. By weight ratio, C: 0.04 to 0.11%, Si: 0.5% or less, Mn: 0.9 to 1.6%, P: 0.03% or less, S: 0. 0.01% or less, V: 0.10 to 0.15%, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, and Di≡0.367√ C (1 + 0.7Si) (1 + 3.3
3Mn) (1 + 0.35Cu) (1 + 0.36Ni)
(1 + 2.16Cr) (1 + 3Mo) (1 + 1.7V)
In the range of (1 + 1.77Al) = 1.5 to 3.5, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.30%, Ni: 0.05 to 2.0%, Cu: 0.05 to 1.5% of 1 type or 2 types or more, Nb: 0.01 to 0.05%, Ca: 0.001 to 0.006% of 1 type or 2 types, and After hot rolling, the steel containing the balance Fe and unavoidable impurities and substantially not containing B is reheated to a temperature range of Ac _{3 to} 1000 ° C. to be quenched, and further heated to 750 to 850 ° C. again, A method for producing a high yield steel having a low yield ratio of 590 N / mm ^{2 and} excellent in weldability, having a plate thickness of 50 mm or more, characterized by performing quenching and subsequently tempering in a temperature range of Ac ₁ transformation point or less.