JPH02270934A - High tensile strength steel having excellent stress relief annealing embrittlement resistance in heat affected zone - Google Patents

Abstract

PURPOSE:To obtain the high tensile strength steel for welded structure having excellent stress relief annealing embrittlement resistance by limiting each amt. of alloy elements to be added and limiting their total amt. to be added. CONSTITUTION:The above high tensile strength steel for welded structure contains, by weight, <=0.18% C, <=0.70% Si, <=2.0% Mo, <=2.0% Mn, <=0.020% P and 0.005 to 0.10% Al, according to the require for strength and toughness, furthermore contains one or more kinds among <=0.7% Cu, <=2.0% Ni, <=1.0% Cr, <=1.0% Mo, <=0.2% V, <=0.05% Nb, <=0.03% Ti and <=0.015% B, which has the strong point of satisfying the inequality. According to the fig., in the steel satisfying the limited componental area in the above-mentioned manner and having <=2.0% Ceq (S) value, toughness of <=-35 deg.C vTr6 is secured even after subjected to heating treatment corresponding to stress relief annealing. Thus, it is clear that the steel has stress relief annealing resistance.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to high tensile strength steel used in steel structures that require stress relief annealing after welding, particularly for members that require toughness in welded areas. It is.

(Prior Art) Several proposals have been made for the purpose of preventing stress relief annealing embrittlement of steel materials used for the above-mentioned purposes. For example, JP-A-52-%19, JP-A-54-458
Publication No. 12 describes an improvement by reducing P or N in steel as much as possible, which is considered to be the main cause of embrittlement.
No. 2-9820 proposes improvements by incorporating special elements such as rare earth elements and Ca into steel.

However, these proposals are not necessarily sufficient in terms of preventing embrittlement and have drawbacks such as increasing the manufacturing cost of steel materials, and new techniques are required.

(Problems to be Solved by the Invention) The present invention proposes a new stress-relieving annealed embrittlement steel as a solution to the above-mentioned current situation.

Conventionally, since the weld heat-affected coarse grain zone is exposed to high temperatures, the alloying elements in the steel are in a solid solution state, and these precipitate during stress relief annealing, resulting in so-called precipitation embrittlement, which deteriorates toughness, or It is known that impurity elements that are unavoidably contained in H are segregated at grain boundaries of coarse prior austenite during stress relief annealing, resulting in significant deterioration of toughness mainly due to so-called temper brittleness.

As measures to prevent these embrittlements, proposals have been made to limit the amount of precipitated embrittlement elements in steel and to reduce the amount of impurity elements that cause temper embrittlement, as described above.

However, the former measure is difficult to apply to high-strength steel, and furthermore, there are problems in that it is difficult to expect the effect of reducing impurity elements in high-strength steel of 70 kg or more.

In view of the current situation, the present inventors conducted extensive research to develop high-strength steel that does not deteriorate in toughness after stress-relieving annealing, and as a result, we have developed measures that ignore economic efficiency that have been taken up until now. By limiting the amount of alloying elements and the combination thereof, we propose a high-strength steel with excellent toughness after stress relief annealing.

(Means and effects for solving the problem) The present invention has been made in this way, and its gist is that C:O, 1.8% or less in weight percent, S and O
, 70% or less, Mn: 2.0% or less, P: 0.020
% or less, Δρ: 0.005% to 0. IO% Further, according to the requirements for strength and toughness, Cu: o, 7% or less, N I: 2.0
% or less, C: 0.0% or less, Mo: 0.0% or less, V: 0.2% or less, Nl++ 0.05% or less, TI
: 0.03% or less, B : 0.015% or less, the balance consists of Fe and impurities, and at the same time C
eq(S)=C+:0.9SI+0.64Mn+42P
+0.2Cu+0.2Ni+0.4Cr+1Mo+10
It is a high tensile steel with excellent stress relief annealing embrittlement properties in the weld heat affected zone, which satisfies Nb+17TI≦2.0%.

The present invention will be explained in detail below.

First, the reason why the alloying element content was limited to the above range will be described.

C is added to ensure strength, but if it exceeds 0.18%, the toughness, weldability, and stress relief annealing embrittlement properties of the steel deteriorate, so 0.18% is set as the upper limit.

Si requires a content of 0.03% or more to ensure strength and deoxidize, but if it exceeds 0.70%, toughness deteriorates, so this was set as the upper limit.

Mn is added to ensure strength, but if it exceeds 2.0%, weldability and stress relief annealing embrittlement properties deteriorate, so this is set as the upper limit.

If P is added in an amount exceeding 0.02%, the stress relief annealing embrittlement property, which is an effect of the present invention, cannot be exhibited, so this is set as the upper limit.

AΩ is 0, which is the range normally used as a deoxidizing element.
, 005 to 0.100%.

Cu, Ni, Cr, Mo, V, Nb, Ti, and B are each essential addition elements for the purpose of improving strength and toughness.
If added in excess of the above-mentioned limited range, weldability and stress relief annealing embrittlement properties would deteriorate, so this was set as the upper limit.

In the present invention, as described above, the amount of each alloying element added is limited, and the total amount added by combining these elements is: C
The effect is exhibited only when the eq(S) value satisfies a specific formula, and this point will be explained below using experimental results.

The Ccq(S) formula performs a welding reproduction thermal cycle (maximum heating temperature: 1400℃, cooling time 800℃ on 60 steel types).
~500°C for 10 seconds), then heated at 580°C/3H and 50°C/H, which corresponds to stress relief annealing.
The toughness after the same treatment was evaluated by a Charpy test, and the correlation with the chemical composition was determined by multiple regression calculation.

The reason why the welding thermal cycle conditions were determined to be the above conditions with a fast cooling rate is because it is known that the decrease in toughness due to stress relief annealing is greater as the structure of the weld heat affected zone becomes a low-temperature transformed structure. be.

In addition, if excellent toughness is obtained after stress relief annealing under these conditions, actual welded joints will be multilayer welded, so there will be fewer embrittled areas at the notch tip, and there will be weld metal mixed in, among other factors. Therefore, it is inevitable that a higher joint toughness can be obtained.

Among the experimental results, Ceq(S) value is 1, 7~2, 4%
Figure 1 shows the data within the range.

According to FIG. 1, Cc
Steel with q(S) value: 2.0% or less maintains toughness of vTrs: -35℃ or less even after heat treatment equivalent to stress relief annealing,
It is clear that it has excellent stress relief annealing properties.

(Example) Table 1 shows the composition, Ceq(S), base metal strength, and toughness value of the welding simulated thermal cycle material after stress relief annealing: vTrs of the steel used.

As is clear from the table, it is clear that the steel of the present invention has superior toughness after stress relief annealing compared to the comparative steel.

(Effects of the Invention) As is clear from the above explanation, by limiting the amount of each alloying element added as well as the total amount of these elements added, a welded structure with excellent stress relief annealing embrittlement properties can be obtained. It is clear that it is possible to produce high-strength steel for industrial use. Therefore, it can be said that the present invention has great industrial effects.

[Brief explanation of drawings]

FIG. 1 is a chart showing the relationship between Ceq(S) value and toughness after stress relief annealing. Agent Patent Attorney Tate Chanogi Mistake 1 Figure f, 6:0. B 2.0 2.
2 2.4 (% Ce No. (S)

Claims (1)

[Claims] C: 0.18% or less Si: 0.70% or less Mn: 2.0% or less P: 0.020% or less Al: 0.005% to 0.10% In addition, strength Depending on toughness requirements, Cu: 0.7% or less Ni: 2.0% or less Cr: 1.0% or less Mo: 1.0% or less V: 0.2% or less Nb: 0.05% or less Ti : 0.03% or less B: Contains one or more types of 0.015% or less, the balance consists of Fe and impurities, and at the same time Ceq(S) = C + 1.9Si + 0.64Mn + 42P
+0.2Cu+0.2Ni+0.4Cr+1Mo+10
A high-strength steel with excellent stress relief annealing embrittlement properties in a weld heat affected zone, characterized by satisfying Nb+17Ti≦2.0%.