JP3497250B2 - 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.

[0002]

2. Description of the Related Art Conventionally, a thick (sheet thickness of 50 mm or more) 590
N / mm ² class high-strength steel (hereinafter referred to as HT60) was manufactured by quenching and tempering steel having high hardenability by adding B or adding a large amount of various alloys. However, as described above, with the addition of B and the addition of a large amount of alloy, the weldability is remarkably inferior, and at the time of on-site welding, preheating of 100 ° C. or more is required from the viewpoint of welding crack prevention, and the work efficiency is significantly reduced and the work environment is reduced. It was getting worse.

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

[0004]

SUMMARY OF THE INVENTION The present invention is an HT6 for construction.
It provides a low cost manufacturing technology. The HT60 having a plate thickness of 50 mm to 100 mm manufactured based on the method of the present invention is
Weld cracks are less likely to occur even in small heat input welding or restraint welding, and preheating can be reduced or omitted during welding work. Buildings using this HT60 absorb earthquake energy and have excellent earthquake resistance. It has features such as having.

[0005]

Means for Solving the Problems The present invention overcomes the above-mentioned problems and achieves the object, and its concrete means are shown in (1) and (2) below.

(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: 0.01% or less, Nb: 0.01 to 0.05%, V: 0.01 to 0.06 %, Ti: 0.005 to 0.025 %, Al: 0.018 to 0.1 %, N: 0.006% or less, and in the range of Di shown by the following formula: Cr: 0.05 to 0.50%, Mo: 0.05 to 0.30%, Ni: 0.05 to 2.0% , Cu: 0.05 to 0.47 % 1 type or 2 types or more, and the balance is steel containing iron and unavoidable impurities and containing substantially no B 1100.
Reheat to the temperature range above ℃ ℃ 1250 ℃, 900
After rolling such that the cumulative rolling reduction of 30 ° C or less is 30% or more, quenching is immediately performed from a temperature of 750 ° C or more to room temperature, and reheating and quenching is performed in a temperature range of 720 to 850 ° C.
A method for producing a low yield ratio 590 N / mm ² class high-strength steel with excellent weldability, which comprises performing tempering treatment in a temperature range not higher than the Ac ₁ transformation point.

[Formula]

(2) Further, by weight ratio, Ca: 0.001 to 0.006%
And a low yield ratio of 590 N / mm ² class high-strength steel having excellent weldability and having a thickness of 50 mm or more and 100 mm or less.

[0007]

The present invention will be described below. According to the research conducted by the inventors, in order to simultaneously realize excellent weldability and low yield ratio of thick (sheet thickness 50 mm or more) HT60, B-free steel,
It was found that it is necessary to control the precipitation hardening and further the composition of the steel to an appropriate hardenability. In addition, small amount of Ti addition,
It was found that the steel with the refined crystal grains by optimizing the process conditions exhibits excellent low temperature toughness even when precipitation hardening is performed. The minimum amounts of Nb and V required to obtain the characteristics of HT60 are 0.01% each. However, if it exceeds 0.05% and 0.06%, respectively, it becomes difficult to sufficiently reduce YR, so this was made the upper limit.

Further, in order to keep YR below a predetermined value (80%), it is necessary to adjust the hardenability of steel to an appropriate range, and the index represented by Di shown in the following equation is used. It is necessary to adjust it in the range of 1.5 to 3.0. The reason is 1.
This is because if it is less than 5, YR is not sufficiently reduced by the two-phase structured heat treatment described later, and if it exceeds 3.0, HT60 is over-strength and weldability is deteriorated.

Next, in order to sufficiently exert the precipitation hardening as described above and sufficiently lower YR, the manufacturing method must be appropriate. Reheating temperature is 1100 ° C or more and 1250 ° C
The reason for limiting the temperature range to the following is to keep the austenite grains during heating small and to make the rolling structure finer. 1250 ° C is the upper limit temperature at which the austenite grains do not become extremely coarse during heating, and if the heating temperature exceeds this temperature, the austenite grains become coarse mixed grains and the structure after transformation becomes a coarse bainite structure, so the toughness of the steel is increased. Is significantly deteriorated. The slab heated under the conditions as described above is rolled so that the cumulative reduction amount in the unrecrystallized region of 900 ° C. or less is 30% or more. This is because the austenite grains are made finer by rolling in the unrecrystallized region.

Next, regarding the cooling conditions after rolling, after completion of rolling, the steel sheet is quenched from a temperature of 750 ° C. or higher to room temperature. This is because if the quenching temperature after completion of rolling is less than 750 ° C, transformation proceeds and predetermined strength cannot be obtained. Then 7
After reheating to 20 to 850 ° C., quenching, reheating to a temperature of Ac ₁ or lower, and tempering treatment. The reason for reheating and quenching to 720 to 850 ° C. is to reduce the yield ratio. Generally, precipitation hardened steel has a significantly high YR. Then 720
Reheat and quench in the (γ + α) two-phase region at 850 ° C.
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. When the reheating temperature is 720 ° C. or lower, the above-described effect cannot be obtained because the region that transforms to γ is small. However, if the temperature exceeds 850 ° C., most of the γ-transformation occurs and the desired two-phase structure cannot be obtained, so that low YR cannot be achieved.

The tempering treatment is essential for exerting precipitation hardening. However, if the temperature exceeds the Ac ₁ point, the strength is remarkably reduced, so the temperature must be set to the Ac ₁ point or lower (the desirable tempering temperature is 450 to 650 ° C.).
However, even if the amount of Nb and V added and the manufacturing method are appropriate,
If the basic components are not suitable, excellent properties as HT60 cannot be obtained.

Hereinafter, this point will be described. The lower limit of 0.04% of C is to secure the strength of the base metal and the welded portion and N
It is the minimum amount for exerting these effects when b and V are added. However, if the amount of C is too large, the weldability is significantly deteriorated, so the upper limit was made 0.1%. Addition of a large amount of Si deteriorates the weldability and HAZ toughness, so the upper limit is set to 0.
It was set to 5%. Only Al and Ti are sufficient for deoxidizing steel, and Si is not necessarily added.

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

Cr is an element that enhances the strength of the base material and the welded portion, and is required to be at least 0.05% or more. However, if too large, the weldability and HAZ toughness are 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 its upper limit is 0.30%. Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but the effect is weak at less than 0.05% and exceeds 2.0% .
Since obtain lose economical to become very expensive, the upper limit was set at 2.0%.

Cu has almost the same effect as Ni,
Growth and corrosion resistance of the strength by Cu, to improved weather resistance has an effect. In this case, if the Cu content exceeds 0.47 %,
The effect is saturated, and if less than 0.05%, there is no effect, so the Cu content is limited to 0.05 to 0.47 %. Ti forms a carbonitride and improves HAZ toughness. When the amount of Al is small, an oxide of Ti is formed to improve the HAZ toughness, but if it is less than 0.005%, it has no effect, and 0.025%.
%, There is an unfavorable effect on the HAZ toughness, so the content is limited to 0.005-0.025%.

Al is an element generally contained in deoxidized upper steel ,
The lower limit is 0.018% . However, if the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the welded portion deteriorates, so the upper limit was made 0.1%. N is generally contained in steel as an unavoidable impurity, but it is combined with Nb and V to form a carbonitride to increase strength, and TiN is formed to improve the properties of HT60 as described above. Increase. 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. However, in order to further improve the characteristics for the object, the following element, that is, Ca, is selectively added to improve the strength and toughness. With respect to, more preferable results are obtained. Next, the additional element and the amount of addition will be described.

Ca controls the morphology of sulfide (MnS),
It has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, CaO, C
A large amount of aS is generated and becomes large inclusions, which not only impairs the toughness of steel but also the cleanliness and adversely affects the weldability and lamella tear resistance. Therefore, the range of Ca addition amount is 0.001 to 0.0
It is set to 06%. The method of the present invention is most preferably applied to a thick plate mill, but can also be applied to hot coils, shaped steel and the like.

[0019]

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. Inventive steels 7-1 in Tables 1-6
7 shows the chemical composition of comparative steel. Table 2 shows the steel sheet manufacturing conditions of the present invention steel and the comparative steel, and their mechanical properties and weldability.

[0020]

[Table 1]

[0021]

[Table 2]

In the steels 1 to 4 and 6 of the present invention shown in Table 2, the strength, YR, toughness and weldability of the base material can be achieved in a well-balanced manner. In contrast, Comparative Steel 7 has a low Di value, so Y
R is high. Comparative Steel 8 has a high C content, and the y slit crack stop temperature is extremely high. In Comparative Steel 9, since B is added, the y slit crack stop temperature is extremely high. Comparative steel 10 has a low Nb content and lacks strength. Comparative steel 11 has a low V content and lacks strength. Further, in Comparative Steel 12, since the Di value is too high, the strength is extremely high and exceeds the standard strength of HT60.

In Comparative Steel 13, the reheating temperature during quenching in the two-phase region is as low as 700 ° C., the γ conversion is insufficient, YR is high, and the y slit crack stop temperature is also very high. In Comparative Steel 14, the heating temperature of the slab was as high as 1280 ° C. and the crystal grains were coarsened, so the base material toughness was poor.
In Comparative Steel 15, the cumulative rolling reduction at 900 ° C. or lower is low and the base material toughness is poor. In Comparative Steel 16, the water cooling start temperature immediately after rolling is low and the strength is insufficient. In Comparative Steel 17, the reheating temperature at the time of quenching in the two-phase region is as high as 870 ° C., so that most of it was converted to γ and the YR was high.

[0024]

EFFECTS OF THE INVENTION Steel materials such as thick steel plates, shaped steels and hot coils manufactured by the chemical composition and manufacturing method of 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.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-7-242941 (JP, A) JP-A-61-10043 (JP, A) JP-A-2-205626 (JP, A) JP-A-2- 282418 (JP, A) JP-A-5-179343 (JP, A) Patent 3007246 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 6/00 C21D 8/00-8 / 10 C21D 9/00 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A weight ratio of 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, Nb: 0.01 to 0.05%, V: 0.01 to 0.06 %, Ti: 0.005 to 0.025 %, Al: 0.018 to 0.1 %, N: 0.006% or less, and within the range of Di shown by the following formula: Cr: 0.05 to 0.50%, Mo: 0.05 to 0.30%, Ni: 0.05 to 2.0%, Cu : it contains one or more from 0.05 to 0.47%, the steel containing substantially no B and the balance being iron and unavoidable impurities 1100
Reheat to the temperature range above ℃ ℃ 1250 ℃, 900
After rolling so that the cumulative rolling reduction of 30 ° C or less is 30% or more, immediately quenching from a temperature of 750 ° C or more to room temperature, reheating to a temperature range of 720 to 850 ° C, quenching,
A method for producing a low yield ratio 590 N / mm ² class high-strength steel with excellent weldability, which comprises performing tempering treatment in a temperature range not higher than the Ac ₁ transformation point. [Formula] 2. The production of a low yield ratio 590 N / mm ² class high strength steel with excellent weldability according to claim 1, further containing Ca: 0.001 to 0.006% by weight. Method.