JPS61560A - Steel having superior toughness at welded joint - Google Patents

Abstract

PURPOSE:To obtain a steel having superior toughness at a welded joint at a low cost by adding specified percentages of C, Si, Mn, V, Ti, Nb and sol.Al to Fe. CONSTITUTION:A steel consisting of, by weight, 0.05-0.20% C, 0.05-0.50% Si, 0.50-2.00% Mn, 0.005-0.025% V, 0.005-0.015% Ti, 0.005-0.025% Nb, 0.010- 0.040% sol.Al and the balance Fe or further contg. one or more among <=0.70% Ni, <=0.30% Mo and <=0.50% Cu is manufactured. The steel has 40-60kgf/mm.<2> tensile strength and gives a steel sheet having such high toughness as TNOT-30 deg.C or below at a welded joint.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to steel with excellent weld joint toughness.

(Conventional technology) In recent years, steel structures such as pressure vessels, reactor containment vessels, and offshore structures are being used in colder regions and safety standards have become more stringent. Toughness values are now required.

Furthermore, toughness is often required for welded joints in addition to the base metal. For toughness testing, in addition to the conventional Charpy test, ASTM E2
There are an increasing number of cases in which the TNDT (non-ductile transition temperature) determined from the thin-film test specified in 2008 is regulated.

Conventionally, as a means of improving the base material toughness of this type of steel plate, a method of grain refinement through controlled rolling has been adopted for steel for line pipes and the like. On the other hand, for improving the toughness of welded joints, for example, iron and adjustment 61 (1975) No. 11, 65-7
A method of adding Ti to a steel for high heat input welding, page 9, a method of adding REM and B to a steel for high heat input welding, and a method of adding REM and B to a steel for high heat input welding, and a method of adding REM and B to a steel for high heat input welding, and a method of adding Ti to a steel for high heat input welding, and a method of adding Ti to a steel for high heat input welding, and a method of adding Ti to a steel for high heat input welding, and a method of adding Ti to a steel for high heat input welding. Monthly Iron and Steel Association 103rd Lecture Conference'82-8'638,56
39, a method of adding Ti and B in combination to steel for high heat input welding was proposed, and it was reported that the toughness of the joint was improved.

However, in all of these methods, the toughness is evaluated based on the absorbed energy of the Charpy test, and the TNDT of the joint is
I can't find any research on this.

Therefore, the reality is that there is no established theory as to what should be done to secure TNDT.

(Problems to be Solved by the Invention) The present invention solves these problems by combining appropriate steel components, not only in the base metal but also in the welded joint in particular.
In addition to the Yalpie test, our objective is to provide a steel that satisfies the following high-level toughness for TNDT, which has been in high demand recently: TNDT-aot.

(Structure of the invention) The present invention aims to solve the above problems.

As a result of various experiments, (1) weight c: o, os ~ 0
．． 20%, Si: 0.05-0.50%, Mn=0
．． 50-2.00%, V: 0.0054-0
．． 025%, Ti: 0.005-0.015%, N
b: 0.005-0.025%, Sol, Al
: 0.01 (1-0,040%, balance Fe and unavoidable impurities) A steel with excellent weld joint toughness.

(2) In (1) above, Ni: 0.70% or less, Mo
Cu: 0.30% or less, Cu: 0.50% or less, and is constructed as a steel with excellent weld joint toughness.

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

C is necessary for maintaining strength, and if it is less than 0.05%, it will be difficult to ensure strength. On the other hand, the upper limit is set at 0.20% since addition of a large amount has a negative effect on the toughness of the welded joint. Si is effective as a deoxidizing element, and in order to obtain this effect, 0.05
% or more is necessary. If a large amount of Si is added, the toughness of the welded joint will be impaired, so the upper limit is set at 0.50%.

Calf is an element that is effective in securing strength and improving toughness.
．． It is necessary to add 50% or more, but if it exceeds 2.00%, the effect becomes very small, so 2.00% is the upper limit.

Ti is an important element in the composition of the present invention along with Nb, .

That is, although the addition of Ti has little effect on the toughness of the base metal, it significantly improves the toughness of the welded joint, especially TNDT. As shown in Figure 2, this effect occurs when the amount of Ti is 0.
．． 0.005% or more, but if it exceeds 0.015%, TNDT actually deteriorates. Therefore, the appropriate range is set to 0.
005 to 0.015%.

Nb has the effect of refining the structure, and this effect is effective in improving the base metal strength, toughness, and TNDT of the welded joint. The effect can be seen at tR above o, oos%, but if it increases, the toughness of the welded joint will deteriorate, so the upper limit should be set at 0.
025%.

■ is effective in improving the strength of the base material, and this effect can be achieved with an addition amount of 0.
．． It is recognized at 0.05% or more, but if it increases, it deteriorates the TNDT of the welded joint, so it should be kept at 0.025% or less.

Sol and Al are essential for improving the toughness of the base material through grain refinement, and their effects are exhibited at 0.010% or more. On the other hand, if it exceeds 0.040%, the toughness will decrease due to the formation of inclusions, so the upper limit is set to 0.040%.

The steel of the present invention is mainly used as a material for pressure vessels, nuclear reactor containment vessels, marine structures, etc., and has a tensile strength of 40 to 60 k.
In the low strength range, including gf/we'' class steels, the above basic ingredients are sufficient, but in the case of thick plates, it is necessary to add appropriate reinforcing ingredients and select a heat treatment method to obtain high strength. The type and amount of such reinforcing components must be determined taking into consideration that they do not deteriorate the joint toughness of the welded joint of steel and do not have a negative effect on weldability.In the present invention, as the component N less than 0.7%
i, Molo of 0.30% or less, and Cu of 5% or less. These ingredients may be used singly or in combination within their respective limited amounts.
When added in combination, they contribute to increasing the strength of steel.

Although Ni is effective in improving strength, it does not need to exceed 0.7%, and since it is an expensive element, it is set to 0.7% or less from cost considerations.

Mo is particularly effective in ensuring the strength of the base material after PWHT (post-weld heat treatment), but if added in a large amount, the toughness of the weld joint will be impaired, so the content should be 0.30% or less. Cu is effective in improving the strength of the base material, but if it exceeds 0.40%, the strength improving effect decreases, so the upper limit is set to 0.40%.

Although P and S as unavoidable impurities are not particularly limited, it is preferable to suppress each to 0.010% or less since they deteriorate the quality of the steel due to segregation and formation of inclusions.

Therefore, the steel of the present invention containing the above components can be used as hot-rolled, or after hot-rolled, normalized, tempered, or quenched-tempered heat treated, and after hot-rolled online control. Cooled acid has excellent weld joint toughness even after heat treatment such as controlled cooling and subsequent recovery.

Therefore, whether or not to perform post-rolling heat treatment, and what kind of heat treatment to perform, can be determined from the viewpoint of mechanical properties such as strength and toughness required of the steel material.

(Action of the Invention) Most conventional studies on improving toughness have been conducted using the Charpy test as an indicator. This Charpy test value and TNDT are said to have a considerable correlation in the base metal, but there are very few studies on welded joints, and from the experience of the present inventors, the correlation between the two is very poor, and the conventional Based on knowledge, it is difficult to improve the TNDT of welded joints.

According to the study by the present inventors, in order to improve the TNDT of welded joints, it is necessary to
It was found that an important point is to improve the crack propagation arresting toughness of the weld heat affected zone (which occurs during welding).

That is, as shown in FIG. 1, the HAZ in which the brittle crack 7 generated from the mechanical notch 1 propagates is classified into two types. The first is the weld heat affected zone (HA) that exists in the quenched state.
ZI), and 5 in the figure is it. The other is HA
ZI is a heat-affected zone (this is referred to as HAZII) that exists in a state where it has been tempered in the next welding pass, and 6 in Fig. 1 is the heat-affected zone.

In the figure, 2 indicates the embrittlement bead of the electric drop test piece formed according to ASTM E208, 3 the weld metal of the welded joint, 4 the base material, and t the thickness of the electric drop test piece.

However, in order to improve the TNDT of the welded joint, H
It is necessary to improve the crack propagation arresting toughness of both AZI and HAZn. For this reason, as a result of examining the influence of various metallurgical factors, the following was found.

That is, addition of Ti is effective for improving the toughness of HAZI. Figure 2 shows C: 0.05-0.20%, Si
: 0.05-0.50%, Mn: 0.50-2.
00%, v20, OO5~0.025%, Nb: 0
．． 005-0.025%, Sol, At: 0.0
Using steel sheets containing various amounts of Ti in the range of 10 to 0.040% and subjected to hot rolling and subsequent conditioning heat treatment, the samples were subjected to a simulated thermal cycle (HAZI) with an equivalent heat input of 45 KJ/lxr. This figure shows the relationship between the press notch Charpy absorbed energy and the amount of Ti added at -60° C. after applying an equivalent amount of Ti.

The reason why press notch Charpy was carried out here is that while V-notch Charpy, which is generally performed, shows the toughness at the time of crack initiation, press notch Charpy shows the toughness at which crack propagation is stopped.

As is clear from the figure, it can be seen that adding Ti in the range of 0.005 to 0.05% improves the toughness. The reason for this is that by adding an appropriate amount of Ti, the structure of HAZI becomes finer, which improves the crack propagation arresting toughness of the core.

It is.

On the other hand, in order to investigate the relationship of metallurgical factors to the improvement of toughness of HAZI, the above-mentioned HAZI sample subjected to a heat input of 45 KJ/(to) equivalent thermal cycle was tempered at 650°C for 5 seconds and then pressed at -60°C. The Notch Charpy absorbed energy was investigated and the results shown in Table 1 were obtained.

Table 1 (Note 1) Heat cycle conditions Two-person heat cycle equivalent to 45 Kj/C + 650°C x 5 sec tempering Note 2) PE-ao knee press notch at 60°C As shown in Figure 2, it is effective to keep the upper limit of the amount of C low and to add Nb and (2) in combination. The reason for this is Nb.

This is because the addition of a small amount of compound (2) causes the carbides of HAZII to be finely dispersed and the structure to be fine-grained, thereby improving crack propagation arresting toughness. And this effect is Nb.

V is well exhibited at 0.05 to 0.025%.

From the above results, Ti: 0.005 to 0.015%,
V: 0.005-0.0025%, Nb: 0.
When added in a controlled manner at 0.005 to 0.025%,
Both AZ I and HAZ II have high crack propagation arrest toughness, and as a result, the welded joints exhibit very excellent TNDT values.

(Example) Table 2 shows the chemical composition of the steel plate obtained by rolling a thick plate, and Table 3 shows the type of heat treatment and the mechanical properties obtained.

As shown in Tables 2 and 3, steels F-H are examples of the first invention of the present application, and steels 1 to N are examples of the second invention of the present application, which have excellent properties exceeding Tuor-aot. The joint has good toughness.

On the other hand, steels 0-Q are comparative examples of the first invention, steels R to U are comparative examples of the second invention, but since steel 0 does not contain Ti and has a high Nb content, steel P has V, Since steel Q does not contain Ti, the V and Nb contents are too high, so both TN at the joint part
DT is bad.

Also, since #sR does not contain V or Ti, steel S is C
Besides having too high a content, it does not contain Ti; steel T has a high C and Nb content and does not contain V; steel U has a high C and No content, as well as 'J, Ti,
Since they do not contain Nb, the TNDT of the joints is poor.

(Effects of the Invention) The present invention has found that the TNDT of welded joints can be improved by adding Ti, Nb, and V in a composite manner to the steel, and it has been found that the TNDT of the welded joint can be improved, which is highly demanded in 40-60 kg3/mm2 class steel. TNDT-3ot: A steel plate with the following high joint toughness can be provided at a low cost, and it has great industrial benefits.

[Brief explanation of drawings]

Figure 1 is a schematic diagram illustrating the relationship between the brittle crack propagation surface and the structure on the propagation surface in an electric drop test of a welded joint. Figure 2 shows the relationship between brittle crack propagation arrest toughness and Ti content in the HAZI part. It is an explanatory diagram. 1... Mechanical notch 2... Brittle bead of electric drop test piece 3... Weld metal of welded joint 4... Base metal 5 - Weld heat affected zone (HAZI) 6... Welding heat Affected zone (HAZII) 7...Brittle crack t...Electrical drop test specimen thickness Figure 1 Opening: HAIZ (Heat affected zone that is proudly produced in 718) Opening: HA'lπ (HAII: It"' Next 1,

784 咄4/Mazutsu"it and ゛saφc-1ite) Figure 2? Ti (%)

Claims (2)

[Claims] (1) C: 0.05-0.20%, Si: 0.0% by weight.
05-0.50%, Mn: 0.50-2.00%, V:
0.005-0.025%, Ti: 0.005-0.0
15%, Nb: 0.005-0.025%, Sol. A
A steel with excellent weld joint toughness characterized by having l: 0.010 to 0.040%, the balance being Fe and unavoidable impurities. (2) C: 0.05-0.20%, Si: 0.0 by weight
5-0.50%, Mn: 0.50-2.00%, V: 0
．． 005-0.025%, Ti: 0.005-0.01
5%, Nb: 0.005-0.025%, Sol. Al
:0.010~0.040%, and Ni:0.70%
Hereinafter, a steel having excellent welded joint toughness characterized by comprising one or more of Mo: 0.30% or less, Cu: 0.50% or less, the balance being Fe and unavoidable impurities.