JPH09104949A - Steel plate for ultralarge heat input welding structure excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a steel plate for welding structures excellent in HAZ toughness in which, in welding environmental conditions in ultralarge heat input welding, TiN having a size not to be melted and dissipated even for a long high temp. residence time characteristic of ultralarge heat input welding is secured in HAZ contg. bonds and fine BN is largely precipitated with the above TiN as the nuclei. SOLUTION: This steel plate has a compsn. contg., be weight, 0.03 to 0.18% C, 0.1 to 1.0% Si, 0.5 to 1.8% Mn, 0.005 to 0.060% Al, 0.005 to 0.025% Ti and 0.0005 to 0.0020% B, contg. N satisfying 0<EN<0.0020 in the formula of EN= N-0.292Ti-1.292B, and the balance Fe with inevitable impurities, and furthermore, TiN precipitates having 0.1 to 1μm diameter equivalent to a circle are present by >=3×10<5> pieces per mm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to 500 kJ / cm.
The toughness of the heat-affected zone including the bond (hereinafter referred to as HAZ) is -20 even when welding with an extremely high heat input of about 1000 kJ / cm.
The present invention relates to a steel plate for welded structure having an excellent value of 4.0 kgf-m or more at ° C.

[0002]

2. Description of the Related Art In recent years, the use of super-high heat input welding is being considered for welding structural steel sheets in order to reduce welding costs.
Also in this case, a steel sheet having excellent HAZ toughness is desired.

Conventionally, in the field of large heat input welding, as a proposal of a steel sheet having good HAZ toughness and a manufacturing method thereof, there are, for example, Japanese Examined Patent Publication No. 55-26164 and Japanese Unexamined Patent Publication No. 63-103051. .

The proposal of Japanese Patent Publication No. 55-26164 is as follows.
As a large heat input welding method, a heat input of 320 kJ / cm or less (electroslag welding) is used as a target, and a fine TiN of 0.02 μm or less is secured in the steel to obtain HAZ austenite grains. Smaller, HAZ
It is characterized by ensuring the toughness of.

However, this proposal has a heat input of 320 kJ / c.
It is intended for welding of m or less, and the guaranteed temperature of HAZ toughness is up to 0 ° C.

Further, the latter proposal of Japanese Patent Laid-Open No. 63-103051 is intended for welding with a heat input of about 230 kJ / cm, and for the same reason as the proposal of Japanese Patent Publication No. 55-26164, 0 Fine Ti of 0.02 to 0.04 μm
By securing the required amount of N in the steel, the HAZ toughness is improved.

On the other hand, 500 kJ / cm to 1000 kJ /
Ultra-high heat input welding of about cm is already possible.

These ultra-high heat input welding methods are disclosed in Japanese Patent Publication No. 55-
26164 and JP-A-63-103051, the amount of heat input is larger than the target heat input, and TiN is melted.
The residence time at 350 ° C or higher is significantly increased. Therefore, fine TiN does not melt at the bond part of super-high heat input welding and the HAZ part and does not bring about the expected effect.
Making toughness difficult.

On the other hand, since the welded structure constructed by using the super-high heat input welding method is larger in size than the welded structure constructed by the high heat input welding method, the safety of the structure is high. HAZ toughness including weld bonds is more severely required due to the magnitude of social and economic impacts, and it is desired to guarantee this to lower temperatures.

[0010]

The problem to be solved by the present invention is to meet this demand, and a welding environment of an extremely large heat input welding of a huge 500 kJ / cm to 1000 kJ / cm which is uncomparable with the conventional heat input. Under the conditions, TiN of a size that does not dissolve and disappear even under the long high temperature residence time peculiar to ultra-high heat input welding is secured in the HAZ including the bond.
HAZ toughness vE by depositing a large amount of fine BN in the core of
It is to provide a steel plate for welded structure in which -20 shows 4.0 kgf-m or more.

[0011]

The ultra-high heat input welding structural steel sheet excellent in low temperature toughness of the present invention has a C: 0.03% by weight.
~ 0.18%, Si: 0.1-1.0%, Mn: 0.5
~ L. 8%, Al: 0.005-0.060%, Ti:
0.005-0.025%, B: 0.0005-0.0
020%, EN = N-0.292Ti-1.2
92B, a TiN precipitate containing N satisfying 0 <EN <0.0020, the balance being Fe and unavoidable impurities, and having a circle equivalent diameter of 0.1 μm or more and 1 μm or less is 1 mm ² It is characterized in that there are 3 × 10 ⁵ or more per unit.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION C is added to secure the strength of the base metal, and the upper limit is to prevent addition of Ti and B which are added separately to form TiC and / or BC to deteriorate the toughness of HAZ. It has established.

Si is added to deoxidize the steel and secure the strength of the base metal, and Mn is added to secure the strength of the base metal.
An upper limit is set to prevent deterioration of the toughness of AZ.

Al is added in order to make the steel effective in matching the crystal orientation of the steel and recrystallization by solid solution and precipitation in the rolling process in addition to the refinement of the steel by Al nitride. However, if the addition amount is small, it has no effect, and if the addition amount is excessive, the toughness of the pot is deteriorated.

[0015] Ti is of a size that does not melt into a weld bond.
N is secured, and this is used as a nucleus to promote reprecipitation of BN, and HAZ
Is added in order to secure the toughness of the HAZ by refining the structure of, and the upper limit of B is regulated from the prevention of the formation of BC. It is provided to prevent the deterioration of

As a result, the HAZ crystal grains become finer and
The refinement of the AZ structure is promoted, and the toughness of the HAZ containing the bond is significantly improved. Therefore, the upper limit is HAZ including bond
The solid solution N due to the melting of AlN in (3) becomes excessive with respect to the upper limit of B determined from the BC generation and causes an excess of N, leaving the solid solution N in the HAZ containing the bond to deteriorate the toughness. It is set to prevent.

B is added in order to secure the bond toughness by refining the HAZ structure after ultra-high heat input welding and fixing the solid solution N. The upper limit improves the hardenability of the HAZ by the formation of BC during reprecipitation. Therefore, the HAZ toughness is prevented from deteriorating.

N is the lower limit of the amount of N necessary to secure TiN of a size that does not melt even at a high temperature and a large heat capacity of an extremely large heat input welding bond with a heat input of about 500 to 1000 kJ / cm, and the upper limit thereof is the above BC. From the upper limit of B for suppressing the formation, T in the steel is prevented in order to prevent N that could not be finally fixed as BN from forming a solid solution and degrading the toughness of the bond.
The upper and lower limits are set according to the i amount and the B amount.

It is desirable that S be as small as possible in order to prevent the inclusions in the steel from forming MnS and deteriorating the toughness of the base material.

Further, Ni, Cr, Mo, Cu, Nb, V
It is not a problem to add one or two or more of them according to their respective action and effects, as is usually used in the art.

Ni is added to improve the toughness of the base metal and HAZ, and the upper limit is preferably determined to enhance hardenability and prevent bainite formation of the HAZ structure.
Mo and Cu are added to improve the strength of the base material,
It is necessary to consider the upper limit in order to prevent the HAZ from hardening.

Nb and V are added to form carbides and nitrides to accelerate the refinement of the crystal grains of the base metal structure and to improve the strength and toughness, but the upper limit is added to prevent deterioration of the HAZ toughness. Caution must be taken.

[0023]

EXAMPLE FIG. 1 shows an example of the measurement results of the thermal cycle in the vicinity of the weld bond portion in the super-high heat input welding and the single-sided single layer high heat input welding of the steel sheet of the present invention. From the figure, 14
It has been found that the residence time of exposure to high temperatures of 00 ° C or higher is significantly longer than in single-sided single-layer welding.

FIG. 2 is a view of the bonding portion 14 at the time of welding.
A residence time of 00 ° C. or higher and a critical dimension of the TiN precipitate that is dissolved at that time (diameter obtained by converting the shape of the precipitate into a circle) are shown.

JP-B-55-26164 and JP-A-6-26
400 targeted by the proposal of JP-A-3-103051
The residence time corresponding to welding of kJ / cm or less is shorter than that of the ultra-high heat input welding targeted by the present invention, so that the TiN precipitate of 0.05 μm or less is present even after being thermally affected. , It is possible to contribute to the miniaturization of the HAZ structure, but it was found that TiN of less than 0.1 μm melts and disappears in ultra-high heat input welding.

Further, FIG. 3 shows the HAZ of the ultra-high heat input welded portion.
The relationship between the toughness and the number of TiN precipitates having a diameter of 0.1 μm or more in the steel sheet portion subjected to the welding cycle is shown for the B-containing steel and the B-free steel.

From this figure, the HAZ toughness is poor in the B-free steel regardless of the number of TiN precipitates, and in the B-containing steel EN = N-0.292Ti-1.295B, 0 <EN < Within the range of 0.0020, the diameter of the unmelted portion after the heat cycle of welding is 0.1 μm.
It was found that the presence of 3 × 10 ⁵ or more of the above TiN precipitates per mm ² ensures the HAZ toughness even under super-high heat input welding. However, if TiN becomes too coarse, it may become the core of brittle fracture.
If the diameter is equivalent to the following circle, it is the target temperature range -20
It was found that the temperature does not become the nucleus of brittle fracture at about ° C.

Therefore, in order to sufficiently secure TiN precipitates having a diameter of 0.1 μm or more and 1 μm or less, the influence of the solidification cooling rate of steel having the chemical composition shown in Table 1 on the size of TiN precipitates was examined. The results are shown in FIGS.

[0029]

[Table 1] As is clear from FIG. 4, at a cooling rate of 5 ° C./min or more, the diameter of precipitated TiN becomes finer than 0.1 μm,
To secure the precipitated TiN with a diameter of 0.1 μm or more, 5 °
It is necessary to solidify at a cooling rate of C / min or less.

Further, from FIG. 5, a steel slab cooled at a cooling rate of more than 5 ° C./min during solidification cooling is 1200 ° C. to 130 ° C.
After reheating to 0 ° C, cooling at a cooling rate of 5 ° C / min or less does not affect the cooling rate during solidification, and
At the time of rolling, perform the normal rolling or set the heating temperature to Ac ₃
By setting the temperature to 1200 ° C, precipitated TiN having a diameter of 0.1 µm or more can be secured until the time of ultra-high heat input welding.

FIG. 6 shows the results of an investigation of the effect of chemical components on the toughness of a super-heat-input welded part using a steel sheet produced by changing the chemical components variously and setting the solidification cooling rate to 5 ° C./min or less. Show.

From this figure, in the steel containing B, N
However, in the formula EN = N-0.292Ti-1.295B, it is clear that a high value of the heat cycle toughness at the peak temperature of 1400 ° C corresponding to the super large heat input welding can be secured in the range of 0 <EN <0.0020. Became.

When the relationship between these HAZ structures and TiN precipitates was investigated, the mechanism for improving the HAZ toughness by TiN precipitates having a diameter of 0.1 μm or more is due to the fine TiN precipitates that have been conventionally known. Not by the effect of preventing austenite coarsening, but by coarse TiN precipitates that remain unmelted and bulk pro-eutectoid ferrite that forms a composite precipitate of BN that precipitates at a temperature directly above the ferrite transformation using this TiN precipitate as a nucleus It has been found.

That is, the presence of this massive pro-eutectoid ferrite suppresses the production of plate-like pro-eutectoid ferrite, and as a result, the ferrite side plate (hereinafter referred to as FSP) develops with this plate-like pro-eutectoid ferrite as a production site. It was found that the HAZ toughness is improved because the production of FSP, which is harmful to the toughness, is suppressed, and the HAZ toughness is improved.

Also in the B-containing steel, if N> 0.0020% in the equation where N is EN = N-0.292Ti-1.295B, it becomes difficult to secure toughness due to excess N, and EN < In the case of 0%, the amount of N required for precipitation of BN cannot be secured, so
Since N cannot be precipitated, solid solution B increases the hardenability of the bond portion, does not generate massive proeutectoid ferrite, develops FSP, and makes it difficult to secure toughness.

FIG. 7 (1) schematically shows the positional relationship and shape of the massive proeutectoid ferrite α1 in the steel sheet of the present invention, which has a fine HAZ structure to improve the HAZ toughness. ) Is the above-mentioned plate-like pro-eutectoid ferrite α2 and FSP and upper bainite B, which form a poor HAZ structure and deteriorate HAZ toughness in the conventional steel sheet.
3 is a schematic diagram showing the positional relationship and shape of u on the tissue.

From the above findings, a steel containing necessary amounts of Ti, N and B was prepared, and the cooling rate during solidification by casting or the cooling rate after remelting and heating TiN was 5 ° C./min or less. By adjusting the size and amount of the TiN precipitate as described above, the TiN precipitate that does not dissolve even in a high temperature and high temperature welding bond formed by an extremely large heat input of 500 to 1000 kJ / cm is secured. By using it as a precipitation site and precipitating BN immediately above the ferrite transformation, it is possible to improve the toughness of the HAZ containing the bond during super-high heat input welding.

Due to the effect of ferritic transformation promotion of BN, massive pro-eutectoid ferrite α1 is generated at every grain boundary,
There is no plate-like pro-eutectoid ferrite α2 and FSP shown in FIG. 7 (2), and a fine HAZ structure composed of fine-grained crystal grains surrounded by massive pro-eutectoid ferrite α1 shown in FIG. 7 (1) is formed. .

The sample steel was cast into a slab by continuous casting, subjected to known control rolling and controlled cooling to give a steel plate, and the obtained steel plates were each subjected to welding under the conditions shown in 4.

The results are shown below.

1. Table 1 Table 1

2. Steel plate manufacturing conditions are shown in Table 2.

3. The mechanical properties of the steel sheet are shown in Table 2.

4. Welding conditions are shown in Table 2.

5. HAZ toughness is shown in Table 2.

[0046]

[Table 2] Steel Nos. 1 to 12 shown in Table 2 show examples of the present invention. In this example, each toughness vE-20 shown by super heat input welding of 500 kJ / cm to 1000 kJ / cm is 4.1 to 20.9 in weld bond (FL) and 6 in HAZ, 1 mm. 0.0-18.0, HAZ, 3 mm 14.3-2
7.2, HAZ, 5 mm was 21.6 to 27.0, and the yield point of the steel sheet was 32 to 48 kgf / mm ² .

On the other hand, steel numbers 13 to 22 show comparative examples. In all of Comparative Examples of Steel Nos. 13 to 17, the cooling rate during solidification or after reheating was 5 ° C./min or more, the number of TiN precipitates of 0.1 μm or more was small, and Steel No. 18 was Ti. Is excessive and N is insufficient. Steel No. 19 has an excessive amount of N. Steel Nos. 20 to 21 have an insufficient amount of N and cannot obtain a sufficient number of TiN precipitates. Steel No. 22 has an excessive amount of B. Met.

In the comparative examples of steel Nos. 13 to 22, the heat input amount, which is the welding condition similar to that of the present invention, is 500 kJ / cm to 100.
Each toughness vE-20 at 0 kJ / cm ultra-high heat input welding is
Weld bond (FL) is 0.4 to 3.4, HA
Z, 1 mm is vE-20 with 2.1 to 4.3, HAZ, 3
mm is 3.5 to 14.0, which is 4.0 kgf-of the task.
The HAZ toughness of m or more was not reached.

[0049]

【The invention's effect】

(1) The steel sheet of the present invention has a heat input amount of 500 kJ / cm to 1
H including a bond of ultra-high heat input welding of about 000 kJ / cm
It is possible to meet the environmental conditions of ultra-high heat input welding, which is applied to AZ at high temperature and long residence time.

(2) Even under an environment of high temperature and long residence time, by securing a required number of TiN precipitates of insoluble size in the HAZ containing weld bonds during welding, synergistic effect of TiN and BN The HAZ toughness including the weld bond is 4.0 kgf-m.
The above excellent low temperature toughness is exhibited.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An example of the thermal cycle measurement result in the vicinity of the weld bond portion in the large layer heat input welding is shown.

FIG. 2 shows a residence time of 1400 ° C. or higher at a bond portion during welding and a critical dimension (diameter) of TiN precipitate which is melted at that time.

[FIG. 3] HAZ toughness of a super-high heat input welded part and T of 0.1 μm or more in diameter in a steel plate part subjected to the welding heat cycle.
The relationship with the number of iN precipitates is shown for B-containing steel and B-free steel.

FIG. 4 shows the effect of cooling rate of steel after casting solidification on the size of TiN precipitates.

FIG. 5 shows the relationship between the cooling rate after reheating and the size of TiN precipitates in a steel piece reheated to a temperature at which TiN partially melts.

FIG. 6 shows the influence of chemical components on the toughness of a super-high heat input weld.

FIG. 7 schematically shows the positional relationship on the structure and the characteristic of shape of massive pro-eutectoid ferrite, IFP and plate ferrite, FSP, Bu and the like. (1) shows the steel plate of the present invention,
(2) shows a conventional steel plate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiaki Hoshi 1 Nishinosu, Oita-shi, Oita-shi Nippon Steel Corporation (72) Inventor Hidesato Mabuchi Oita-shi, Nishinosu 1-Nihon Steel Company Oita Steel Works

Claims (1)

[Claims] 1. C: 0.03 to 0.18% by weight,
Si: 0.1-1.0%, Mn: 0.5-l. 8%, A
1: 0.005-0.060%, Ti: 0.005-
0.025%, B: 0.0005 to 0.0020%, EN = N-0.292Ti-1.292B in the formula, 0 <EN <0.0020 is contained, and the balance is Consists of Fe and inevitable impurities, and
TiN having a circle equivalent diameter of 0.1 μm or more and 1 μm or less
Ultra-high heat input welding structural steel sheet with excellent low temperature toughness, characterized in that there are 3 × 10 ⁵ or more precipitates per 1 mm ² .