JPH05186848A - Steel for large heat input welding excellent in toughness in weld heat-affected zone - Google Patents

Abstract

PURPOSE:To remarkably reduce the cost in the construction of a structure by providing a steel composition consisting of respectively specified percentages of C, Si, Mn, P, S, V, Ti, Al and N and the balance Fe with inevitable impurities. CONSTITUTION:The steel for large heat input welding has a composition consisting of, by weight, 0.03-0.10% C, 0.01-0.5% Si, 0.4-2.0% Mn, <=0.03% P, 0.001-0.01% S, 0.03-0.20% V, 0.005-0.02% Ti, 0.005-0.06% Al, 0.005-0.02% N and the balance Fe with inevitable impurities. As a group of strength and toughness improving elements, either or both of 0.2-1.5% Cu and 0.2-3.0% Ni are incorporated. Further, 0.05-1.0% Cr and/or 0.05-1.0% Mo is incorporated. By this method, the number of stages in welding can be remarkably reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel having excellent weld heat affected zone toughness.

[0002]

2. Description of the Related Art Efficient welding by increasing heat input at the time of welding has a great advantage in reducing the construction cost of a structure. However, as the welding heat input increases, the toughness of the weld heat-affected zone (hereinafter abbreviated as HAZ) of the steel material decreases, and therefore, studies have hitherto been made on a steel sheet with a small decrease in toughness during high heat input welding. ..

For example, in a large heat input steel developed for shipbuilding, marine structures, etc. as disclosed in Japanese Patent Laid-Open No. 58-31065, the grain size is large due to a nitride such as TiN. The toughness is improved by suppressing the deterioration. However, since TiN is melted at the boundary between the weld metal and the HAZ, it is difficult to stably secure high toughness over a wide area of the HAZ.

Therefore, in recent years, Japanese Unexamined Patent Publication No. 62-1842.
As disclosed in the publication, deoxidation product + TiN +
There is a proposal to positively reduce the size of ferrite grains by generating ferrite in the cooling process at the time of welding by using a complex precipitate of MnS as a transformation nucleus. In addition, JP-A-59-1
Japanese Patent No. 85760 and Japanese Patent Application Laid-Open No. 60-245768 have already disclosed inventions based on the same technical idea such as utilizing a composite precipitate of Ti oxide and nitride and MnS as a transformation nucleus.

However, in the steel for large heat input utilizing these deoxidation products or oxides, in the production thereof,
A high-level manufacturing technique is required to uniformly disperse the oxides in the steel sheet, which makes it difficult to stably manufacture a large number of steel sheets.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a rope for large heat input welding in which the HAZ toughness is remarkably reduced even at the time of large heat input welding, and moreover, stable supply in large quantities is possible without requiring advanced manufacturing technology. To do.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have studied various transformation nuclei in order to cause ferrite transformation which is generated in the cooling process during welding. The formation of fine ferrite and its effect are further promoted by the formation of TiN. Further, if precipitation of V carbide occurs during cooling, HAZ is significantly hardened and toughness is lowered, so the C content is reduced. Revealed the need.

The present invention will be described in detail below. A technique for improving the strength and toughness of steel by adding N at the same time as V has been known. For example, Japanese Examined Patent Publication No. 39-2368 discloses "V at least 0.02%, N 0.08% to 0.24%, C 0.15% to 0.50%, Mn 0.
High-strength semi-killed steel containing 40% to 2.00% and treated with an amount of deoxidizing agent insufficient to produce killed steel. "Also, Japanese Patent Publication No. 48-13803. "C: 0.08 to 0.20%, Si: 0.20
~ 0.40%, Mn: 1.0-1.6%, Cr: 0.0
5 to 0.35%, Mo: 0.10 to 0.60%, V:
0.005-0.1%, N2: 0.004-0.02%
A low-alloy steel for high temperature, characterized in that as a final heat treatment, normalizing is performed at a cooling rate of 1000 ° C to 4500 ° C / h from a heating temperature of 890 ° C to 1000 ° C. " No. 61-130420 discloses "C:
0.05 to 0.20%, Si: 0.01 to 0.6%, M
n: 1.10 to 1.6%, V: 0.03 to 0.2%,
N: 0.009 to 0.02% of steel is forged into a flange shape, followed by quenching and tempering, and the like "is disclosed.

In any of these inventions, the feature is the addition of V and N, whereby a large amount of VN is precipitated, and the grain size is made finer and strengthened. Since VN has a low melting temperature, it melts during heating by normal leveling and quenching, and then precipitates during the cooling process with air cooling or during the tempering process after quenching, strengthening the steel base material. Let

However, from the viewpoint of effective utilization of VN, the above-mentioned prior art is premised on not adding Ti which forms a nitride, which improves the mechanical properties of the base material, but has a high HAZ toughness. However, there is a problem in that the crystal grains are coarsened. As an exception, JP-A-56-
No. 127750 discloses "C: 0.12-0.25.
%, Mn: 0.5 to 2.9%, V: 0.05 to 0.20
%, Cu: 0.05 to 0.4%, Ni: 0.1 to 0.5
%, Cr: 0.05 to 0.4%, Ti: 0.005 to
0.03%, N: 0.01 to 0.03%, Ca: 0.
0005-0.0070%, Mg: 0.0005-0.
There is a description that Ti is added at the same time as V is added as "a structural high-strength steel containing at least one or two kinds of 0070% and having less strain-age embrittlement".

However, the addition of Ti in this case is for the purpose of improving the strength, and it is necessary to add Ti in the range of 0.12 to 0.20% C.
Therefore, a remarkable effect cannot be expected to improve the HAZ toughness, which is not the object of the present invention.

The present invention has examined the above-mentioned problems of the prior art utilizing VN, and in order to effectively utilize the precipitation of VN with respect to the HAZ toughness, at the same time as limiting the addition amounts of V and N, C
It was experimentally found that the toughness can be dramatically improved by suppressing the addition amount and further adding Ti.

FIG. 1 shows 0.05% C-0.25% Si-
1.0% Mn-0.002% P-0.003% S-0.
Based on 068% V, toughness was evaluated as a transition temperature in a Charpy impact test when a heat cycle imitating a large heat input welding of 1000 kJ / cm at a maximum heating temperature of 1400 ° C. when changing N was applied. It is a thing. In the figure, Ti is 0.
The case where 010 to 0.015 wt% is added and the case where no addition is made are shown.

As is clear from the figure, the amount of N added is 0.0
When the content is 05 wt% or more, the HAZ toughness is remarkably improved regardless of whether Ti is added or not, but the toughness is further improved by adding Ti.

VN has a good lattice matching with ferrite and precipitates in austenite to generate a large amount of fine ferrite from within the austenite grains. In that case, in order to improve the toughness, the grain size is increased. It is important to effectively realize the fine dispersion of ferrite generated inside. Recent studies have known that VN precipitates with MnS as nuclei.

Therefore, the present inventors have found that various experiments have shown that M
It was clarified that nS preferentially precipitates in TiN generated at higher temperature, and TiN was finely dispersed by adding Ti, and TiN-MnS-VN composite precipitates could be dispersed in the steel to improve HAZ toughness. We have found that it can be improved significantly.

However, even if the refinement of ferrite is achieved, if V carbides that precipitate at a temperature lower than VN precipitate, the toughness is significantly impaired. Therefore, it was also found that in order to suppress the formation of this carbide, it is also important to suppress the amount of C added.

The present invention is constructed on the basis of the above findings, and the gist thereof is C: 0.03 to 0.10%, S
i: 0.01 to 0.5%, Mn: 0.4 to 2.0%,
P: 0.030% or less, S: 0.001-0.01%,
V: 0.03 to 0.20%, Ti: 0.005 to 0.0
20%, Al: 0.005-0.06%, N: 0.00
5 to 0.02% and further Cu: 0.2 to 1.5
%, Ni: 0.2 to 3.0% or Cr: 0.05 to
It is a steel for high heat input welding in which 1.0% and Mo: 0.05 to 1.0% each of one type or two or more types are added.

[0019]

The operation of the present invention will be described in detail below. First, the reason why the steel composition is limited as described above in the present invention will be described.

C: C is an element necessary for securing strength,
It is necessary to add 0.03%, but if it exceeds 0.10%, precipitation of V carbides will occur and the HAZ toughness will decrease, so the content range is made 0.03 to 0.10%.

Si: Si is an element necessary as a deoxidizing element for steelmaking, and 0.01% is contained in steel, but 0.5
%, The base material and HAZ toughness deteriorate. Therefore, the range is set to 0.01 to 0.5%.

Mn: Mn is an element necessary for ensuring strength and toughness. However, if it exceeds 2.0%, the HAZ toughness is remarkably lowered, and conversely, if it is less than 0.4%, it becomes difficult to secure the strength of the base material, so the range is limited to 0.4 to 2.0%. ..

P: P is an intergranular embrittlement element and it is desirable to reduce it as much as possible. However, since the degree of embrittlement is small at 0.03 wt% or less, the upper limit is made 0.03 wt%.

S: S is an element indispensable for the formation of MnS which acts as a VN precipitation nucleus in the present invention.
It is necessary to add more than 01%, but a large amount of MnS
Therefore, the upper limit is 0.0.
10%.

V: V is an element that plays a central role in the present invention, and as described above, promotes ferrite transformation during cooling after welding and improves HAZ toughness. Therefore, it is necessary to add 0.03% or more,
Addition of a large amount causes precipitation of carbides, so the content is limited to 0.20% or less.

Ti: Ti is an indispensable element in order to promote the refinement of crystal grains by TiN and the fine dispersion of MnS which becomes a VN precipitation nucleus, and it is necessary to add 0.005% or more. However, since excessive addition of carbides may cause carbides to deteriorate in toughness, its upper limit is 0.020.
Limit to%.

Al: Al is an element necessary as a deoxidizing agent, and it is necessary to add 0.005% or more. However, if it is added excessively, AlN is excessively generated, and N effective for generating VN is generated. The upper limit is set to 0.06% because it may decrease.

N: N is necessary for the production of TiN and VN, and as shown in FIG.
It is necessary to add 005% or more, but an excessive addition causes deterioration of the base metal and HAZ toughness, so the upper limit is 0.0
2%

In the present invention, the effect of the present invention is not impaired even if the following elements are selectively added as a strength / toughness improving element or a strength improving element. First, one or two of the following can be selected and added as an element for improving strength and toughness.

Cu: Cu is effective as an element for improving toughness and strength, and its effect is effective when added in an amount of 0.2% or more, but excessive addition of more than 1.5% causes deterioration of toughness. The upper limit is set to 1.5% in order to bring about.

Ni: Ni is an element effective for improving the toughness and needs to be added in an amount of 0.2% or more. However, if it exceeds 3.0%, the effect is saturated, so 3.0%. Is the upper limit.

Further, as the strength improving element, one or two of Cr and Mo can be added. 0.0 respectively
It is necessary to add 5% or more, but excessive addition impairs toughness, so the range is limited to 1.0% in all cases.

In the production of a steel sheet, steel having the above component system is melted in a converter, an electric furnace or the like, and a steel slab is cast by continuous casting or ingot agglomeration. After that, heating the plate,
Hot rolling is performed to manufacture a steel plate having a predetermined thickness.

Regarding the manufacturing conditions after heating the thick plate, the properties of HAZ are not affected even if various techniques such as controlled rolling and controlled cooling that are now known are applied.
Further, in order to improve the mechanical properties of the base material, an appropriate heat treatment may be applied after the hot rolling without any problem in the HAZ toughness.

[0035]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter,
The steel pieces obtained by the ingot-agglomeration method or the continuous casting method were heated to 1150 to 1250 ° C., respectively, and then hot-rolled and heat-treated as shown in the table to carry out a steel sheet having a thickness of 25 to 90 mm.

Then, in order to investigate the HAZ toughness of the high heat input weld, the maximum heating temperature corresponding to the welding heat input of 1000 kJ / cm was 1400 ° C., and the cooling time from 800 to 500 ° C. was 75.
A Charpy impact test was performed after a 0 second thermal cycle. Table 2 shows the test results.

[0037]

[Table 1]

[Table 2]

Steels of the present invention (A, E, G, H, I, J, K,
The impact values after thermal cycling of L) all show good values. On the other hand, steels B, C, D and F are outside the scope of the present invention. Among them, Steel B has a C content deviating from the present invention, and the impact value after thermal cycling is remarkably reduced. Steel C has a Ti content, and steel D has a V content outside the scope of the invention. Also in this case, a decrease in impact value after the thermal cycle is recognized. Furthermore, steel F
Is when N is lower than the scope of the present invention. Also in this case V
The amount of N generated is small, and the toughness of the welding heat shadow area is reduced.

[0039]

The steel sheet having the composition range of the present invention has a very good toughness in the heat-affected zone even if large heat input welding is performed. A significant cost reduction can be realized.

[Brief description of drawings]

FIG. 1 0.05% C-0.25% Si-1.0% Mn
Based on -0.002% P-0.003% S-0.068% V, in Ti-added (0.010-0.015%) and additive-free steel, the maximum heating temperature was 1400 when N was changed. It is a chart which evaluated toughness as a transition temperature in a Charpy impact test when a heat cycle imitating a large heat input welding corresponding to a maximum heating temperature of 1000 kJ / cm at 0 ° C was applied.

Claims (3)

[Claims] 1. C .: 0.03 to 0.10% Si: 0.01 to 0.5% Mn: 0.4 to 2.0% P: 0.030% or less S: 0.0. 001-0.01% V: 0.03-0.20% Ti: 0.005-0.020% Al: 0.005-0.06% N: 0.005-0.02% The balance is Fe and High heat input welding steel with excellent toughness in the weld heat affected zone consisting of inevitable impurities. 2. The strength / toughness improving element group according to claim 1, containing 1 or 2 by weight% Cu: 0.2 to 1.5% Ni: 0.2 to 3.0%. High heat input welding steel with excellent toughness. 3. The strength-improving element group contains, by weight%, one or two of Cr: 0.05 to 1.0% Mo: 0.05 to 1.0%. High heat input welding steel with excellent toughness.