JP3522647B2 - Thick 600MPa grade steel with excellent toughness in the heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat affected zone (Heat Affected) when high heat input welding is applied.
Zone: HAZ) with excellent toughness and tensile strength of 600M
It is a Pa class steel material and is used for various welded steel structures such as buildings and bridges.

The present invention is characterized in that it can be applied to a 600 MPa class steel material having a plate thickness of 60 to 100 mm to which electroslag welding is applied.

[0003]

2. Description of the Related Art In HAZ, the heating temperature during welding becomes higher as it gets closer to the melting line.
Heating austenite (γ) in the region heated above ℃
Is significantly coarsened, and the HAZ structure after cooling is coarsened and the toughness deteriorates. The HAZ structure becomes coarser and the HAZ becomes more brittle as the high-efficiency welding having a large heat input such as electrogas welding and electroslag welding is applied. In addition, in a steel component having a tensile strength of 600 MPa, a HAZ structure mainly composed of upper bainite is formed due to the hardenability of the component. The upper bainite is a local embrittlement phase MA (Martensite-Aust).
It is well known that the structure is poor in toughness because it contains a large amount of (enite Constituent: sometimes called island martensite). That is, 6
When high heat input welding is applied to a 00 MPa class steel material, the HAZ structure is constituted by the coarse upper bainite, and the HAZ toughness is significantly deteriorated.

As a technique for improving electroslag welding HAZ toughness of 600 MPa grade steel, R & D Kobe Steel Making Technique, Vol. 46, No. 3 (Dec. 1996),
p. The following two are described in 9-12. The type of HAZ structure is changed from a main body of upper bainite to a main body of ferrite by reducing the Ceq of the steel component. The HAZ structure is refined by adding Ti.

The specifications of the 600 MPa class steel material that can be achieved by the above technology are that the plate thickness is 60 mm or less and the welding heat input is 70 kJ.
/ Mm or less, the average value of the Charpy absorbed energy (vE (0 ° C)) at 0 ° C at the fusion line (FL, bond) is about 70 J / cm ² , which can be read from the above-mentioned literature. However, as a matter of fact, a thick material up to about 100 mm is used for a 600 MPa class steel plate for a box pillar of a high-rise building. Further, the welding heat input amount of the box pillar diaphragm welded portion may reach 100 kJ / mm when the diaphragm thickness is 60 to 80 mm. From the viewpoint of fracture resistance, it is preferable that the HAZ toughness in the vicinity of the fusion line, which is the most embrittled portion, is higher. For example, Charpy absorbed energy at 0 ° C. is desired to be 100 J / cm ² or more. Such thickening, high heat input welding, high HA
A 600 MPa class steel material that balances Z toughness at a high level has not been developed.

[0006]

DISCLOSURE OF THE INVENTION The present invention has a maximum plate thickness of 100 mm, a tensile strength of 600 MPa class, a welding heat input of 100 kJ / mm, and a HAZ toughness near the melting line at 0 ° C. Charpy absorbed energy. 100J on average
It is to provide a steel material having a density of at least 1 / cm ² .

[0007]

The present invention, in% by mass,
C: 0.08 to 0.15%, Si: 0.4% or less, M
n: 1.0 to 1.8%, P: 0.015% or less, S:
0.006% or less, Nb: 0.005 to 0.05%, A
1: 0.001-0.05%, Ti: 0.007-0.
025%, Mg: 0.0003 to 0.005%, B:
0.0003 to 0.003%, O: 0.001 to 0.0
04%, N: 0.001 to 0.005%, Ti / N, which is the ratio of the mass% of Ti to N, is 3 to 6, and Ca: 0.0003 to 0, if necessary. .004
%, REM: 0.0003 to 0.01%, Zr: 0.0
003-0.01%, V: 0.005-0.1%, C
u: 0.05 to 0.5%, Ni: 0.05 to 0.5%,
Cr: 0.05-0.5%, Mo: 0.05-0.5%
The total Tc of the alloy components calculated by the formula (1) using mass% is less than 0.5%, and the balance has a chemical component consisting of iron and unavoidable impurities. Then
0.01 to 0.5 containing an oxide composed of Mg and Al
It is a 600 MPa grade steel material having excellent weld heat-affected zone toughness, in which 1 μm of TiN is present at 10000 pieces / mm ² or more. Tc = Cu + Ni + Cr + Mo ... (1)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the guidelines for improving the HAZ toughness of 600 MPa class steel are limited to the following two points. (1) The type of HAZ structure is changed from an upper bainite-based material containing a large amount of MA to a ferrite-based material containing no MA. (2) The HAZ structure is refined.

While securing the strength of the base material with a plate thickness of 100 mm,
The point of the present invention is how to achieve the above guideline in the HAZ near the melting line with a welding heat input of 100 kJ / mm. The present invention achieves the above two guidelines by making full use of the following three means. (1) is achieved by reducing the addition amounts of Cu, Ni, Cr, and Mo as much as possible. By adding B, (1) and (2) are achieved, and at the same time, the base metal strength is secured. (2) is achieved by adding Mg.

First, the means for achieving the guideline (1) will be described. As a result of studying the ultra-high heat input HAZ of 600 MPa steel having various steel components, Cu, Ni, Cr, and Mo were very much needed to achieve (1) in the thermal history of the HAZ near the fusion line of 100 kJ / mm. I found it to be harmful. In order to make such a HAZ near the melting line a ferrite-based structure, the sum of the mass% of Cu, Ni, Cr, and Mo must be suppressed to at least less than 0.5%. It is preferable to reduce these alloy components as much as possible from the viewpoint of economy and resource saving.

Further, it has been discovered that addition of B is extremely effective as a means for accelerating ferrite transformation in such an extremely high heat input HAZ. B dissolved in γ during welding heating
Segregates at the γ grain boundary during welding cooling, but 100 kJ / mm
Since the cooling rate in the HAZ applied with ultra-high heat input welding is extremely slow, most of the grain boundary segregated B precipitates as iron carbon boride, effectively improving the hardenability of the HAZ near the melting line. I found to lower it. At this time, if the amount of C is too low, the important iron carbon boride is less likely to precipitate, the hardenability is increased, and the upper bainite structure is formed,
The amount of C must be secured within the range described below. B is an essential element in the present invention from the viewpoint of base material strength. The reason is that the plate thickness is 100 without adding Cu, Ni, Cr and Mo.
To secure the base metal strength of 600 MPa class in mm,
This is because it is necessary to build on the base metal by relying on B. In this case, it is necessary to segregate B in the γ grain boundary in a solid solution state to enhance the hardenability, which is completely different from the form of B present in HAZ. In order to secure such B in a solid solution state at the time of assembling the base material, it is necessary to add Ti and fix N as TiN to prevent B from binding with N. When the base material is formed by direct quenching or reheating quenching, the cooling rate thereof is higher than that of the ultra-high heat input HAZ, so iron carbide boride is less likely to be formed. Therefore, when B is used as the base material, it is necessary to pay more attention to precipitation of BN rather than iron carbide boride.

Next, means for achieving the guideline (2) will be described. Even if (1) is achieved by the above means, the HAZ near the fusion line has a major axis of several 1 along the grain boundary of coarse γ grains.
Grain boundary ferrite having a grain size of 00 μm is generated, which prevents improvement in toughness. Therefore, in addition to achieving (1), it is essential to suppress (pin) the γ grain growth in the HAZ near the fusion line. As a result of diligent study, addition of Mg results in several tens.
A large amount of ultrafine oxide containing Mg and Al with a size of nm is generated, and these become TiN precipitation nuclei, and such a composite precipitation particle exceeds 1400 ° C. HA near the melting line
It was discovered that Z strongly pinned gamma grain growth. If such fine composite particles are present at 10000 particles / mm ² or more, the welding line HAZ with a welding heat input of 100 kJ / mm
However, the γ grains are sufficiently fine-grained to form a fine ferrite structure, and the Charpy absorbed energy at 0 ° C is 100 J /
Reach more than cm ² . The generation of grain boundary ferrite along the grain boundaries of small γ grains makes the major axis finer and improves the toughness of the HAZ near the fusion line. The guideline (2) is achieved by this new finding. Further, the iron carbon boride precipitated in the γ grains of the HAZ near the melting line functions as a nucleus of ferrite transformation, and is therefore effective in refining the ferrite generated in the γ grains, achieving the guideline (2). Contribute to.

Next, the reasons for limiting each chemical component will be described.

C is required to be 0.08% or more in order to deposit an iron carbon boride at a super-high heat input HAZ. When C is less than 0.08%, B does not precipitate as iron carbon boride in the super-high heat input HAZ but becomes a solid solution state, so that the hardenability is increased and the upper bainite-based structure is formed to deteriorate the HAZ toughness. .
However, if the amount of C is too large, the toughness of the base metal and HAZ decreases and the weldability deteriorates, so the upper limit is set to 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 is made 0.4%. Since deoxidation is possible with Al, Ti, and Mg, there is no problem even if Si is reduced. Rather, in order to obtain good HAZ toughness, it is desirable that Si be 0.3% or less.

Since Mn is indispensable for securing the strength and toughness of the base material and the welded portion, 1.0% or more is necessary. But,
If the amount of Mn is too large, the HAZ toughness deteriorates, the slab center segregation is promoted, and the weldability deteriorates. Therefore, the upper limit is 1.8%.
And

P and S are impurity elements in the present invention, and it is necessary to reduce them to 0.015% or less and 0.006% or less, respectively, in order to secure a good base material and HAZ material.

Nb is an element effective for refining the base material structure and improves the strength and toughness of the base material. Also, super heat input H
When the AZ structure is controlled mainly by ferrite, HAZ softening is feared, but the precipitation strengthening by Nb can offset the HAZ softening. In order to exert the above effects, 0.005% or more of Nb is necessary. But N
If the content of b is too large, HAZ will be significantly hardened and the toughness will deteriorate, so the upper limit is made 0.05%.

Al acts as a deoxidizer, and at the same time M
Along with g, an ultrafine oxide of several tens of nm is formed and has a pinning effect in the HAZ. For that purpose, 0.001% or more of Al is required. When Al is less than 0.001%, it is not possible to secure an ultrafine oxide of 10,000 pieces / mm ² or more, and pinning of γ grains becomes insufficient at a HAZ near the fusion line of 100 kJ / mm, improving HAZ toughness. Will be difficult. However, if the amount of Al is too large, the Al-based oxide aggregates and coarsens, which may adversely affect the material of the base material and HAZ. Therefore, the upper limit is made 0.05%.

Ti is fixed to N by forming TiN in combination with N. This prevents B from precipitating as BN at the time of assembling the base material, and prevents B in the solid solution state.
To improve hardenability and improve the strength and toughness of the base metal. In addition, TiN has an ultra-fine (M
g, Al) oxide is compound-precipitated in a size of 0.01 to 0.5 μm with nuclei as the core to develop a strong pinning force in the HAZ near the fusion line and improve the HAZ toughness. In order to exert such an effect of TiN, 0.007% or more of Ti is required. However, if the amount of Ti is too large, TiC precipitates and causes significant HAZ embrittlement, so the upper limit of Ti is 0.025.
%. When Si, Al and Mg are low, Ti can also act as a deoxidizing agent.

Mg plays a very important role in the present invention.
Mg constitutes an ultrafine oxide together with Al and has a pinning effect in the HAZ. For that purpose, 0.0003% or more of Mg is required. If the Mg content is less than 0.0003%, it is not possible to secure 10,000 or more ultrafine oxides per mm ² , and pinning of γ grains becomes insufficient at a HAZ near the fusion line of 100 kJ / mm, improving HAZ toughness. Will be difficult. However, even if Mg exceeds 0.004%, the number of ultrafine oxides is saturated, so the upper limit is made 0.005%.

B is a characteristic element of the present invention together with Mg. B plays a contradictory effect on the hardenability between the base metal and HAZ. That is, at the time of assembling the base material, it becomes a solid solution B to enhance the hardenability and improve the strength and toughness of the thick material. On the other hand, during super-high heat input welding, it precipitates as iron carbide boride to reduce the hardenability of HAZ and promote ferrite transformation, which contributes to the improvement of HAZ toughness. To use B in the base metal, it is necessary to fix N by adding Ti, and to use B in HAZ, it is necessary to secure C that is linked to B. In order to exert the above effects, 0.0003% or more of B is necessary. But B
If it is too large, it becomes difficult to achieve both the effects of the base material and HAZ, so the upper limit is 0.003%.

O constitutes an ultrafine (Mg, Al) oxide and has a pinning effect in the HAZ. For that, 0.
O of 001% or more is required. When O is less than 0.001%, it is not possible to secure more than 10000 / mm ² ultrafine oxides, and HA near the fusion line of 100 kJ / mm.
Z pinning of γ grains becomes insufficient and it becomes difficult to improve the HAZ toughness. However, if the amount of O is too large, large oxides of several μm increase and may act as a starting point of brittle fracture, so the upper limit is made 0.004%.

N produces TiN to produce ultrafine (Mg, A
l) It is complexly precipitated in the oxide and has a pinning effect in the HAZ. For that purpose, 0.001% or more of N is necessary.
When N is less than 0.001%, it is not possible to secure 10,000 or more pinning particles / mm ² and 100 kJ /
In the HAZ near the fusion line of mm, pinning of γ grains becomes insufficient, and it becomes difficult to improve the HAZ toughness. However, if the amount of N is too large, BN tends to precipitate when the base material is built,
Since the amount of solid solution N increases and embrittlement may be promoted, the upper limit is set to 0.005%.

Furthermore, Ti / N, which is the ratio of Ti and N, is 3
It is necessary to control to ~ 6. When Ti / N is less than 3, the amount of N is excessive with respect to Ti, and TiN is formed and the remaining N bonds with B to precipitate BN, which deteriorates the hardenability at the time of assembling the base material. It deteriorates the strength and toughness of the base material. In addition, when solid solution N increases in HAZ, embrittlement occurs. on the other hand,
When Ti / N exceeds 6, Ti is excessive with respect to N,
The Ti remaining after forming TiN precipitates as TiC, and makes the base material and HAZ extremely brittle. Ti / N is 3-6
At this time, the base material and the HAZ material can be well balanced.

Next, the reasons for defining the selective element will be described.

Ca, REM, and Zr can contribute to the improvement of the materials of the base material and HAZ by acting as a deoxidizer and controlling the form of sulfide in combination with S. In order to exert these effects, all the elements are 0.00
03% or more is required. However, if these elements are too much, the oxides and sulfides agglomerate and coarsen, and the base metal and H
Since the material of AZ may be deteriorated, the respective upper limits are made 0.004%, 0.01%, and 0.01%. Here, REM refers to a lanthanoid-based element such as La or Ce, and the above effect can be obtained even if a misch metal containing these elements is added.

V is effective in improving the strength of the base material and HAZ by precipitation strengthening. For that purpose, 0.005% or more is necessary. However, if V is too large, the weldability and HAZ toughness deteriorate, so the upper limit is made 0.1%.

Cu, Ni, Cr, and Mo are effective in improving the strength, toughness, corrosion resistance, and weldability of the base material, and for that purpose, each element requires 0.05% or more. However, these elements enhance the hardenability of the super-high heat input HAZ and promote the formation of upper bainite. Therefore, the upper limits of these elements are each 0.5% or less, and the sum of the mass% of these elements is 0. It is necessary to keep it below 5%. If the sum of these elements exceeds 0.5%, the ultra-high heat input HAZ toughness deteriorates.

The steel material of the present invention is adjusted to a predetermined chemical composition in the steelmaking process of the steel industry, and the continuously cast slab is reheated to variously control each process of rolling, cooling and heat treatment to obtain a thick plate or H plate. Manufactured as shaped steel. In order to obtain a strength of 600 MPa class in a base material having a plate thickness of 100 mm, a manufacturing process such as direct quenching after rolling, accelerated cooling, or reheating quenching is effective. When a low yield ratio is required for building steel, it is sufficient to further subject the γ / α two-phase heat treatment. Further, tempering can adjust strength and toughness. It is also possible to carry out hot charge rolling without reheating the slab. HAZ toughness depends on the dispersion state of the steel components and pinning particles. The dispersed state of the pinned particles does not change significantly during the manufacturing process of the base material. Therefore, super large heat input HAZ
The toughness does not largely depend on the manufacturing process of the base material.

The dispersed state of the inclusions defined in the present invention is quantitatively measured by the following method, for example. Mg and A
0.01 to 0.5 μm T containing an oxide of 1
The number of iN is at least 1 at a magnification of 10,000 to 50,000 times using a transmission electron microscope (TEM) by making a replica sample by extracting it from an arbitrary place on the base material steel plate.
Observing over an area of 000 μm ² or more, the number of TiN having a target size is measured, and this is converted into the number per unit area. At this time, the (Mg, Al) oxide and TiN are identified by composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to the TEM and crystal structure analysis of an electron diffraction image by the TEM. When it is complicated to perform such identification for all complex inclusions to be measured, the following procedure is simply performed. First, the square inclusion is regarded as TiN, and the target T
The number of inclusions existing inside the iN is measured.
Next, composite precipitated TiN whose number was measured by such a method
Of these, at least 10 or more are identified in detail in the above manner, and the ratio of (Mg, Al) oxide and TiN present in a complex manner is calculated. Then, this ratio is multiplied by the number of composite precipitated TiN measured at the beginning. If the carbide in the steel interferes with the above TEM observation,
By heat treatment at 500 ° C. or less, the carbide can be aggregated and coarsened, and the observation of the target composite inclusion can be facilitated.

[0032]

EXAMPLES Table 1 shows the chemical composition of continuously cast steel and the number of 0.01-0.5 μm composite precipitated TiN (pinning particles), and Table 2 shows the plate thickness of the steel plate, the base metal manufacturing method, and the base metal. Indicates material and HAZ toughness.

The steel of the present invention has a plate thickness of 80 to 120 mm, a base material TS of 635 to 700 MPa, and a base material vT.
rs is -50 ° C or lower, and the welding heat input is 80 to 120
The Charpy absorbed energy of the bond portion of the electroslag welded portion of kJ / mm exceeds 100 J / cm ² .

On the other hand, the comparative steel is inferior in the material of the base metal or HAZ because the chemical composition is not appropriate. Steel 7 is C
Is too small, B cannot be precipitated as an iron carbon boride in the superheat HAZ, and the HAZ structure is mainly composed of upper bainite, resulting in poor HAZ toughness. Since Steel 8 has too little Nb, the base metal structure becomes coarse and the toughness is poor. Steel 9 has a small amount of ultrafine (Mg, Al) oxides generated because of too little Al, and the HAZ structure is not refined and the toughness is poor. Steel 1
0 is too little Ti, and steel 15 has too much N, so Ti / N is too small and N is excessive.
Becomes ineffective for hardenability and HAZ causes solid solution N embrittlement,
The material is inferior in both the base metal and HAZ. Steel 11 has too much Ti, and Steel 14 has too little N.
Is too large and Ti becomes excessive, and the base material and HAZ become brittle due to TiC precipitation. Steel 12 contains too little Mg, so the number of ultrafine (Mg, Al) oxides produced is small, and H
The AZ structure is not refined and the toughness is poor. Steel 13 is inferior in the material of the base material and HAZ because B is too small. Steel 16 and Steel 17 are base metal or HA because Ti / N is not appropriate.
The material of Z is inferior. Steel 18 contains too much Cu and Ni, so HAZ becomes the upper bainite-based structure and HAZ
Inferior toughness.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

According to the present invention, it is possible to provide a very thick 600 MPa class steel material having good HAZ toughness even if super-heat-input welding is performed, and welding efficiency and weld joint fracture resistance can be compatible at a high level. It was As a result, the welding construction cost can be significantly reduced, and the safety of the welded structure has been remarkably improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akito Kiyose 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Steel Works (72) Inventor Yu Yoshida 1 Kimitsu, Kimitsu-shi Kimitsu Co., Ltd. Ironworks (72) Inventor Ryuji Uemori 20-1 Shintomi, Futtsu-shi Shin Nippon Steel Co., Ltd. Technology Development Headquarters (56) Reference JP 2000-80436 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.08 to 0.15% by mass%,
Si: 0.4% or less, Mn: 1.0 to 1.8%, P:
0.015% or less, S: 0.006% or less, Nb: 0.
005-0.05%, Al: 0.001-0.05%,
Ti: 0.007 to 0.025%, Mg: 0.0003
~ 0.005%, B: 0.0003 to 0.003%,
O: 0.001 to 0.004%, N: 0.001 to 0.
Containing 0.005%, Ti / N, which is the ratio of the mass% of Ti and N, is 3 to 6, and the balance has a chemical component of iron and inevitable impurities, and an oxide of Mg and Al. 0.01-0.5 μm TiN included is 10,000
Thick 600 MPa class steel with excellent weld heat-affected zone toughness that is present in the number of pieces / mm ² or more. 2. In mass%, further Ca: 0.0003
~ 0.004%, REM: 0.0003-0.01%,
Zr: 0.0003 to 0.01%, V: 0.005 to
0.1%, Cu: 0.05 to 0.5%, Ni: 0.05
~ 0.5%, Cr: 0.05-0.5%, Mo: 0.0
5. The total Tc of alloy components containing at least one of 5 to 0.5% and calculated by the formula (1) using mass% is less than 0.5%. Thick 600 MPa class steel with excellent weld heat affected zone toughness. Tc = Cu + Ni + Cr + Mo ... (1)