JPS6031888B2 - How to improve weld toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to heat treatment of high tensile strength steel with excellent toughness in the heat affected zone (hereinafter referred to as HAZ) during welding.

Generally, the toughness of steel in HAZ is inferior to that of the base metal, which poses a problem in use.

The toughness of the H-side is strongly dependent on the structure, and it is known that, for example, a low carbon lower bainite structure exhibits excellent toughness. However, the structure of HAZ is influenced by the welding method, that is, the amount of heat input during welding, as well as the composition system, and the actual situation is that the range of selection is limited by the required base metal strength, welding method, etc. For example, in order to obtain a low carbon lower bainite structure, it is necessary to add large amounts of Mn, Ni, Mo, etc., the tensile strength of the base metal is usually 70k9/pine or more, and the welding heat input is Limited. In ordinary welding of high-tensile steel with a tensile strength of about 50 to 60 k9/m, for example, suffumerge arc welding of a line pipe of x-6, the HAZ structure is mainly composed of upper bainite.

Among upper bainite structures, especially those in which island-shaped martensite is finely dispersed in acicular ferrite, the grade is significantly lower than that of lower bainite or ferrite-pearlite structures. The main reason why steel with an upper bainite structure has poor rutting properties is thought to be that the highly hard island-like martensite acts as a crack initiation point during fracture.

However, the amount of island martensite, for example,
It is thought that the elements selected according to the desired properties are selected from among the elements represented by the carbon equivalent expressed by the formula, and are almost determined by the components that make up the copper and the cooling rate of the austenitic state (rut). Since strength and carbon equivalent almost correspond, it is generally considered that the amount of island martensite is difficult to control under the welding conditions given for steel of the required strength. Carbon equivalent Ceq is given by the following formula.

Ceq=C+Mn/60(Cr+Mo+V)/50(N
i+Cu)/15(%)...'1'Generally, in H rudders made of 80 kg class high tensile steel, low alloy steel, etc., the control of tin coupling or welding ridges where the core heat temperature was too low was inappropriate. In some cases, cracks may occur in the HAZ.

This is so-called low-temperature cracking that occurs at temperatures below 20,000°C. It is well known that the causes of this are that the structure of the H-side is a martensitic structure, that the amount of diffusible hydrogen in the weld metal is high, and that tensile stress acts on the weld.

In order to prevent these low-temperature cracks, stress removal and smoothing treatment (hereinafter referred to as SR treatment) may be carried out as a heat treatment to recover the drop in ballability caused by residual stress after melting. The main purpose of SR treatment is to remove the stress acting on the welded parts and reduce the hardness due to tempering of the martensitic structure, and the heating rate is set to 2. yo/min
After heating at a slow speed of 500 to 700 qo and holding for 1 to 2 hours depending on the thickness of the welding part,
In reality, the cooling rate is a slow cooling rate of 2° C./min. This SR treatment increases equipment investment and product costs, such as the installation of heating equipment, the complexity of the process, and a decrease in the productivity of the product as a welding section. Furthermore, H of steel containing secondary hardening elements such as V and Nb
In AZ, the hardness increases due to the formation of fine precipitates of carbides through SR treatment, which causes SR cracking, and also causes problems such as so-called SR embrittlement due to the formation of grain boundary arms such as P. Are known. As described above, the SR treatment as a heat treatment after welding is carried out for the purpose of preventing deformation over time by removing internal stress, and does not actively improve the toughness of the HAZ.

Rather, SR cracking or SR embrittlement may occur. In addition, SR treatment is carried out on copper with a tensile strength of 80 kg/m or more, and the 45 to 70 kg tensile strength of the present invention is
Currently, SR treatment is not normally performed on steel having a tensile strength of 9/m2, since the structure after welding is upper bainite and HAZ cracking hardly occurs. Therefore, it has a tensile strength of about 45 to 70k9/masu,
At present, it has not been found that the low-temperature plowability of HAZ can be significantly improved by an extremely effective and simple method to improve the ballability of steel whose HAZ structure has an upper bainite structure during welding. As a result of detailed research into the structural factors that govern the plowability of upper bainite steel, the present inventors discovered that heat treatment after welding significantly improves the low-temperature plowability of upper bainitic steel. .

The characteristics of the present invention are that it has a tensile strength of 45 to 70 k9 at a carbon equivalent of 0.30% to 0.45%, and that the H-rudder structure during welding is made of upper bainite.
For high-strength steel used under welding conditions that result in a structure consisting mainly of acicular ferrite, HAZ
It was found that the rutting property of H
The purpose of this study is to clarify that post-heat treatment decomposes the island-like martensite present in the AZ into cementite and ferrite, thereby reducing the amount of island-like martensite that becomes the point of crack initiation.

This effect is particularly remarkable when the amount of N in the steel is small. Next, details of the present invention and reasons for limiting the heat treatment temperature range will be described.

The carbon equivalent given by the first formula, which is a feature of the present invention, is 0.
The range of 30% or more and 0.45% or less is because the steel that is the object of the present invention has a tensile strength of 45 to 70K9/Iso, and the heat input is 30% to 10%. This is because the HAZ structure becomes upper bainite, and the above carbon equivalent is required for this purpose.

The amount of welding heat input is not an essential condition for the present invention, but it is essential that the HAZ structure consists of upper bainite, particularly acicular ferrite and island martensite. As shown in the drawing, the temperature range for heat treatment is limited to 200°C or higher and 500°C or lower.
The low-temperature plowability of the steel is significantly improved.

Temperatures below 200°C are not sufficient to decompose the island-like martensite into cementite and ferrite, which is a feature of the present invention, and the effect of improving the low-temperature axial properties of the HAZ is small, so the lower limit is set at 200°C. And so.

If the heat treatment temperature exceeds 500°C, the hardness of the base metal and HAZ will decrease due to heat treatment, resulting in a decrease in the required base metal strength and welded joint strength, or steel materials containing secondary hardening elements such as Nb and V. The hardness of the HAZ increases due to heat treatment, which not only deteriorates the low-temperature rutting properties of the HAZ but also reduces the SR.
The upper limit should be set at 500℃ as cracking and SR verticalization may occur.
And so. The heating method according to the present invention may be various heating methods such as gas heating, electric furnace heating, and infrared heating, and is not particularly limited. Furthermore, the heat treatment temperature is 200oo
Because it is heated at a relatively low temperature of 500 qo or more,
There is very little oxidation of the welded parts even in an oxidizing atmosphere, so
There is no need to take any particular equipment or means to prevent oxidation due to heating. Further, there is no need to particularly limit the cooling rate after heating. This is unique as conventional SR processing inherently requires slow cooling. The welded parts targeted by the present invention include line pipe materials for cold regions, welded parts for shipbuilding that require low-temperature balling properties,
These include welded parts for pressure vessels and welded parts for construction. Next, the effects of the present invention will be illustrated by examples.

Examples Table 1 shows the components of the steel materials of Examples of the present invention.

After rapidly heating the steel materials shown in Table 1 to 1,350°C, the welding heat input corresponding to a plate thickness of 2 mm is 33. After cooling, heat treatment was performed in a temperature range of 200 to 600 q○ under cooling conditions such that the ship's side was J/A. After applying the above thermal cycle conditions, a 2-side V/tch impact test was conducted. The drawing shows the heat treatment temperature (℃
) is plotted on the horizontal axis and the fracture surface transition temperature in the impact test is plotted on the vertical axis.
The heat treatment significantly lowers the fracture surface transition temperature and significantly improves low-temperature ballability.

Steel A is a steel containing Nb, but the heat treatment temperature is 60°C.
000, the surface transition temperature becomes significantly high and it is clearly seen that the low temperature ballability is poor. As explained in detail above, the method of improving the plowability of a welded joint according to the present invention is extremely economical and effective.

Table 1

[Brief explanation of the drawing]

The drawing shows the relationship between the heat treatment temperature of steels A, B, C, and D and the fracture surface transition temperature of the two-side V impact test.

Claims (1)

[Claims] 1 C%+1/6Mn%+1/5Cr%+1/5Mo%
It has a component such that the carbon equivalent given by +1/5V% +1/15Ni% +1/15Cu% is 0.30% or more and 0.45% or less, and the structure of the heat affected zone after welding is acicular ferrite and islands. A method for improving the toughness of a welded joint, which comprises heating high-strength steel having a mixed structure with martensite to a temperature range of 200°C or higher to 500°C after welding, and then cooling it.