JPH05279789A - Super-high heat input welding structural steel plate with excellent low temperature toughness - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a steel plate for welded structure, which is excellent in toughness of a heat-affected zone including a bond even in super-high heat input welding of about 800 kJ / cm. [Structure] C, Si, Mn, P, S, B, Ti, Al,
Containing a prescribed amount of N, and 0.05 μm or more and 0.1 μm
TiN having a circle equivalent diameter of less than and greater than or equal to 0.1 μm
The presence of a predetermined number or more of the precipitates prevents TiN precipitates that do not dissolve even when subjected to the heat cycle of ultra-high heat input welding from suppressing coarsening of austenite grains, and further precipitates with TiN precipitates as nuclei of MnS and BN. Massive proeutectoid ferrite is formed with the composite precipitate as transformation nuclei. By suppressing the formation of plate-like pro-eutectoid ferrite in which the massive ferrite is harmful to toughness and the ferrite side plate generated from the plate-like ferrite, excellent toughness can be obtained in the super-high heat input welded portion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is 500 kJ / cm-1.
Even in the ultra-high heat input welding of about 000 kJ / cm, the toughness of the heat-affected zone including the bond (hereinafter referred to as HAZ) (−20)
The present invention relates to a steel plate for welded structure having an excellent Charpy absorbed energy at ° C. (Hereinafter referred to as vE- ₂₀ ) of 39 J or more.

[0002]

2. Description of the Related Art In recent years, in order to reduce welding costs, it has been considered to employ super-high heat input welding such as electrogas welding and electroslag welding for steel plates for welded structures. In this case as well, excellent HAZ toughness is obtained. A steel sheet having the same is desired.
Conventionally, in the field of large heat input welding, as a proposal of a steel sheet exhibiting good HAZ toughness, for example, Japanese Patent Publication No. 55-2616.
4 and JP-A-63-103051. The proposal in Japanese Examined Patent Publication No. 55-26164 is intended for a welding method with a heat input of 320 kJ / cm or less (electroslag welding) as a large heat input welding method.
By securing fine TiN of less than μm in steel,
The feature is that the austenite grains of the HAZ are reduced to secure the toughness of the HAZ. However, this proposal is intended for welding with a heat input of 320 kJ / cm or less, and the guaranteed temperature of HAZ toughness is up to 0 ° C.

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 is 0 for the same reason as the proposal of Japanese Patent Publication No. 55-26164. Fine T of 0.02 to 0.04 μm
By securing the required amount of iN in the steel, the HAZ toughness is improved. However, the current situation is already 500-
Ultra-high heat input welding of about 1000 kJ / cm is possible. These ultra-high heat input welding methods are disclosed in Japanese Patent Publication No. 55-261.
No. 64, JP-A No. 63-103051, the heat input is larger than the target, and the TiN melts 1350.
The residence time above ℃ has increased significantly. Therefore, fine Ti is used in the bond part and the HAZ part of ultra-high heat input welding.
N melts and does not bring about the expected effect, making it difficult to secure HAZ toughness. On the other hand, since the welded structure constructed using the ultra-high heat input welding method is larger than the welded structure constructed using the high heat input welding method, the social impact of the safety of the structure In addition, HAZ toughness including weld bonds is more severely required due to the large economic impact, and it is desired to guarantee even lower temperatures.

[0004]

DISCLOSURE OF THE INVENTION The present invention meets the above-mentioned demands, and has an enormous 500 which is incomparable with the conventional heat input.
HAZ containing fine TiN bond that does not melt and disappear even under the long welding residence time peculiar to ultra-high heat input welding under welding environmental conditions of ultra-high heat input welding of about 1000 kJ / cm
It is an object of the present invention to provide a steel plate for welded structure which has a HAZ toughness vE _-20 of 39 J or more and a manufacturing method thereof.

[0005]

According to the present invention, in order to achieve the above-mentioned object, C: 0.03 to 0.18% Si: 0.05 to 0.5% Mn: 0.5% by weight. -1.8% P: <= 0.02% S: 0.001-0.003% B: 0.0003-0.002% Ti: 0.005-0.025% Al: 0.005-0. In the formula EN = N-0.292Ti-1.295B, the content of N is 0 <EN <0.0020 and the balance is iron and other unavoidable components. 1 per 1 mm ^{2 of} TiN precipitate having a circle equivalent diameter of 0.05 μm or more and less than 0.1 μm
× 10 ⁶ or more, T having a circle equivalent diameter of 0.1 μm or more
iN precipitates are present in an amount of 1 × 10 ⁵ or more per 1 mm ^2, and a super-high heat input welding structural steel sheet excellent in low temperature toughness is used as a first means, and C: 0.05 to 0.2 in weight%. % Si: 0.05 to 0.5% Mn: 0.5 to 2.0% P: ≤0.02% S: 0.001 to 0.003% B: 0.0003 to 0.002% Ti: 0.005 to 0.025% Al: 0.005 to 0.060% is contained, and in the formula EN = N-0.292Ti-1.295B, N satisfying 0 <EN <0.0020 is contained. , Ni: ≦ 1.0% Cu: ≦ 1.0% Nb: ≦ 0.05% V: ≦ 0.1% One or more kinds are contained, and the balance is iron and other unavoidable components. And consists of 0.05 μm or more and 0.1
characterized in that TiN precipitates having a circle equivalent diameter of less than μm is 1 mm ² per 1 × 10 ⁶ or more, TiN precipitates having a circle equivalent diameter of more than 0.1μm is present 1 × 10 ⁵ or more per 1 mm ² The ultra-high heat input welded structural steel sheet excellent in low temperature toughness is used as the second means.

The components of the steel sheet of the present invention and the reasons for limiting the amounts of the components will be described below. C is added to secure the strength of the base material, and the lower limit is 0.05% from the strength required for the application, and 0.2% is the upper limit to prevent deterioration of HAZ toughness, weld crack resistance, and welding resistance. I am trying. Si is added in an amount of 0.05% or more to deoxidize the steel and secure the strength of the base metal.
The upper limit is 0.5% to prevent deterioration of toughness due to hardening. Mn is added in an amount of 0.5% or more in order to secure the strength of the base material and to form MnS which serves as a nucleus for forming a precipitate that serves as a nucleus for ferrite transformation in the HAZ portion. % Is the upper limit.

The upper limit of P is 0.02% in order to prevent deterioration of the toughness and cracking resistance of the HAZ due to microsegregation. S
Is 0.001% in order to form MnS which is a nucleus for forming a precipitate which becomes a nucleus of ferrite transformation in the HAZ part, and the lower limit is 0.001%, which is 0.0 to prevent deterioration of the toughness and anisotropy of the base material.
The upper limit is 03%. Al is added in order to effectively work for matching of crystal orientation of steel and recrystallization by solid solution and precipitation in the rolling process in addition to refinement of steel by Al nitride. However, when the addition amount is small, it has no effect, and when it is excessive, the toughness of the steel is deteriorated, so the upper and lower limits are set to 0.060% and 0.005%, respectively.

First, Ti is produced as a Ti oxide during deoxidation, and a large number of Ti nitrides are finely dispersed and precipitated with the Ti oxide as a nucleus. This Ti nitride acts as a growth control of austenite grains and as a nucleation site for precipitates that serve as nuclei for ferrite transformation. In order to obtain these effects, the lower limit of the final content is 0.005%, and when the content is excessive, the toughness is rather deteriorated, and the upper limit is 0.025%. B is added in an amount of 0.0003% or more in order to secure the bond toughness by fixing the solid solution N by refining the HAZ structure after ultra-high heat input welding, and the upper limit improves the hardenability of the HAZ by increasing the solid solution B. In order to prevent the HAZ toughness from deteriorating, 0.00
It is set at 2%. N is 0.004% as a lower limit as the amount of N necessary to secure TiN, and the upper limit is finally B.
In order to prevent N that could not be fixed as N from forming a solid solution and degrading the toughness of the bond, it is set to 0.008%.
Further, it is possible to add one or more of Ni, Cu, Nb, and V according to their respective action and effects, as is commonly used in the art.

Ni is added to improve the toughness of the base metal and HAZ, and the upper limit is set to 1.0% to enhance the hardenability and prevent bainite of the HAZ structure. C
u is added to improve the strength of the base material, but the upper limit is H
It is set to 1.0% to prevent the hardening of AZ. N
b and V are added to improve the hardenability and to form carbides and nitrides to promote the grain refinement of the base metal structure, thereby enhancing the strength and low temperature toughness of the base metal, but preventing the deterioration of the HAZ toughness. Therefore, the upper limits are 0.05% and 0.1%, respectively.

[0010]

The inventors of the present invention repeatedly conducted various experimental studies in order to solve the above problems and obtained the findings shown in FIGS. 1 to 5. The knowledge obtained based on the drawings will be described below. FIG. 1 shows the measurement results of the thermal cycle in the immediate vicinity of the weld bond portion in the ultra-high heat input welding and the single-sided single-layer high heat input welding which are the objects of the present invention. From the figure, it was revealed that the residence time of the ultra-high heat input welding method exposed to a high temperature of 1400 ° C. or higher is significantly longer than that of single-sided single-layer welding.
FIG. 2 shows the residence time of 1400 ° C. or higher at the bond portion during welding and the critical dimension of the TiN precipitate that is dissolved at that time (diameter obtained by converting the shape of the precipitate into a circle). JP-B-55-26164 and JP-A-63-1
400kJ / targeted by the proposal of 03051 publication
Since the residence time corresponding to welding of cm or less is shorter than that of the ultra-high heat input welding targeted by the present invention,
It was found that TiN precipitates with a size of 05 μm or less are present even after being thermally affected and can contribute to the refinement of the HAZ structure, but in super-high heat input welding, TiN with a size of less than 0.1 μm dissolves and disappears. As a result of further detailed investigation, 0.05 μ
It was found that a part of the TiN precipitate having a size of m or more and less than 0.1 μm does not coagulate and grow in the heating process of welding.

FIG. 3 shows the HAZ toughness of the ultra-high heat input weld, the number of TiN precipitates with a diameter of 0.1 μm or more, and 0.05 μm.
It shows the relationship between the number of TiN precipitates of m or more and less than 0.1 μm. From this figure, Ti with a diameter of 0.1 μm or more
N precipitates are present at 1 × 10 ⁵ or more per 1 mm ² ,
TiN precipitates of 0.05 μm or more and less than 0.1 μm are 1 m
It was found that the HAZ toughness is ensured even under ultra-high heat input welding when 1 × 10 ⁶ or more per m ^{2 is} present. FIG. 4 shows the results of investigation on the influence of chemical components on the toughness of the super-high heat input weld. From this figure, in the steel containing B, N
In the formula of EN = N-0.292Ti-1.295B, in the range of 0 <EN <0.0020,
It has been clarified that a high toughness of the ultra-high heat input weld can be secured.

When the relationship between these HAZ structures and precipitates was investigated, the mechanism for improving HAZ toughness by TiN precipitates having a diameter of 0.05 μm or more was that TiN precipitates that did not dissolve even when subjected to the heat cycle of ultra-high heat input welding. In addition to the effect of suppressing austenite coarsening due to the above, by the formation of massive pro-eutectoid ferrite by using MnS and BN composite precipitates which transform into the precipitation precipitates of MnS and BN, which precipitates from the TiN precipitates that have undergone cohesive growth under the heat cycle of ultra-high heat input welding. 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), which is detrimental to the toughness, which develops using this plate-like pro-eutectoid ferrite as a production site. It was found that the HAZ toughness is improved due to the suppression of the formation of ().

FIG. 5 (1) schematically shows the positional relationship and shape on the structure of the above-mentioned massive ferrite α1 which forms a fine HAZ structure and improves the HAZ toughness in the steel sheet of the present invention. (2) is HA that is inferior to the conventional steel sheet
The above-mentioned plate-like proeutectoid ferrite α2 and FSP and upper bainite Bu whose Z structure has deteriorated HAZ toughness.
FIG. 3 is a schematic view showing the positional relationship and the shape on the tissue.
In order to obtain a predetermined number of the TiN precipitates of 0.05 μm or more and less than 0.1 μm and the TiN precipitates of 0.1 μm or more, it is difficult to achieve in a normal process, and Ti in the RH refining
It was found that this can be achieved by adding Al after deoxidizing with. Furthermore, it was found that the effect of this Ti deoxidizing Al addition step can be obtained regardless of the timing of addition of alloying elements other than Ti and Al and the cooling conditions during continuous casting. In this way, it was possible to obtain good HAZ toughness even when the number of TiN precipitates of 0.1 μm or more was small in ultra-high heat input welding with a heat input of 500 to 1000 kJ / cm.

[0014]

EXAMPLES Table 1 shows the chemical composition of the present invention steel and comparative steel, and Table 2 shows the refining conditions in RH. Table 3 shows the continuous casting conditions, the steel plate rolling conditions, the number of TiN precipitates, and Table 4 shows the mechanical properties of the steel plates and the HAZ toughness at the time of high heat input welding and super high heat input welding. No. 1 of the present invention example. 1 to 16 are all good 500 to
It exhibited an HAZ toughness of 1000 kJ / cm, which is an extremely high heat input weld. No. of the comparative example. 17 to 21 is 0.05 μm or more and 0.
The number of precipitates of less than 1 μm is small and does not reach the toughness value of 39J. No. of the comparative example. Nos. 22 to 25 have a small number of precipitates of 0.1 μm or more and do not reach the toughness value of 39J.
No. of the comparative example. Nos. 26 to 30 have ingredients out of the range, and 3
It does not reach the HAZ toughness value of 9J.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

According to the present invention, the heat input amount is 500 to 1000 kJ.
In addition to the effect of suppressing the γ grain growth of the TiN precipitates that remain undissolved even when subjected to the thermal history of ultra-high heat input welding of about 1 / cm, MnS precipitates and BN that precipitate using cohesively grown TiN precipitates as nuclei
The complex precipitates of the precipitates are used as transformation nuclei to finely form massive proeutectoid ferrite, so that H containing weld bonds is produced.
It makes it possible to manufacture a super-high heat input welding structural steel sheet having an AZ toughness of 39 J or more and excellent in low temperature toughness, and the effect of improving safety, reliability, and economic efficiency brought to the relevant field is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a result of measurement of heat 0 cycle in the vicinity of a weld bond portion in super-high heat input welding targeted for the present invention and single-sided single-layer high heat input welding;

FIG. 2 is a diagram showing a residence time of 1400 ° C. or higher at a weld bond portion during welding and a critical dimension of a TiN precipitate which is melted at that time;

[Fig. 3] HAZ toughness of ultra-high heat input weld and diameter of 0.1μ
.mu.m or more TiN precipitates and 0.05 .mu.m or more.
Diagram showing the relationship between the number of TiN precipitates less than 1 μm,

FIG. 4 is a diagram showing the influence of chemical components on the toughness of a super high heat input weld;

FIG. 5 is a diagram showing the outline of the positional relationship and the shape feature of a textured structure such as massive ferrite, IFP and plate ferrite, FSP, and Bu.

Front page continuation (72) Inventor Tadashi Ishikawa Oita-shi, Oita 1st Nishinosu Nippon Steel Co., Ltd. Inside Oita Steel Co., Ltd. (72) Inventor Go Yoneda 1st Nishinosu Oita-shi Oita Pref. Nippon Steel Oita Steel Works

Claims (2)

[Claims] 1. C: 0.03 to 0.18% Si: 0.05 to 0.5% Mn: 0.5 to 1.8% P: ≤ 0.02% S: 0.001 -0.003% B: 0.0003-0.002% Ti: 0.005-0.025% Al: 0.005-0.060% is contained, EN = N-0.292Ti-1.295B In the formula, a TiN precipitate containing N satisfying 0 <EN <0.0020, the balance being iron and other unavoidable components, and having a circle equivalent diameter of 0.05 μm or more and less than 0.1 μm is formed. 1 per 1 mm ²
× 10 ⁶ or more, T having a circle equivalent diameter of 0.1 μm or more
A super large heat input welding structural steel sheet having excellent low temperature toughness, characterized in that 1 × 10 ⁵ or more iN precipitates are present per 1 mm ² . 2. By weight%, C: 0.05 to 0.2% Si: 0.05 to 0.5% Mn: 0.5 to 2.0% P: ≦ 0.02% S: 0.001 -0.003% B: 0.0003-0.002% Ti: 0.005-0.025% Al: 0.005-0.060% is contained, EN = N-0.292Ti-1.295B In the formula, N that satisfies 0 <EN <0.0020 is contained, and further Ni: ≦ 1.0% Cu: ≦ 1.0% Nb: ≦ 0.05% V: ≦ 0.1% Alternatively, it contains two or more kinds, and the balance consists of iron and other unavoidable components, and is 0.05 μm or more and 0.1 or more.
characterized in that TiN precipitates having a circle equivalent diameter of less than μm is 1 mm ² per 1 × 10 ⁶ or more, TiN precipitates having a circle equivalent diameter of more than 0.1μm is present 1 × 10 ⁵ or more per 1 mm ² Ultra-high heat input welding structural steel plate with excellent low temperature toughness.