JPS5989753A - Thick-walled steel for low temperature use with superior strength and superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To improve considerably the toughness of a thick-walled base metal and a weld zone by restricting the amounts of C, Cu, Cr, Mo and Nb+V contained in 3.5% Ni steel for low temp. use having a specified composition and by heat treating the steel to form a fine bainite structure. CONSTITUTION:A steel consisting of, by weight, 0.03-0.05% C, 0.15-0.30% Si, 0.40-1.00% Mn, 3.0-4.0% Ni, 0.01-0.04% sol.Al, 0.10-0.30% Cu, 0.10-0.25% Mo, 0.10-0.30% Cr, >0-0.01% Nb+V and the balance Fe is subjected to normalizing, hardening and tempering, or other similar heat treatment to form a fine bainite structure. The resulting thick-walled steel for low temp. use has superior toughness at its base metal and weld zone when used as the material of a thick product.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thick-walled low-temperature steel with excellent strength and low-temperature toughness, especially for chemical equipment such as LPG storage tanks, ethylene plants or coal liquefaction plants.
This is intended to advantageously combine strength and low-temperature toughness for so-called low-temperature steel materials that are used in the extremely low temperature range of 5°C to -110°C.

Conventionally, chemical equipment such as ethylene plants and coal liquefaction plants has a temperature of -100℃ to -110℃8,
5% Ni steel is used, but the tensile strength is 50-70
For thick materials such as 50 mm or more in the kg f/Ml'' class, it is difficult to ensure sufficient toughness, and this is due not only to the level of the base material itself, but also to the embrittlement caused by SR after welding. Furthermore, the toughness level of the welded part, both in the heat-affected zone of the bond part and the heat-affected zone, remains at an extremely low level compared to the level of the base metal, regardless of the plate thickness.

Therefore, the present invention is primarily intended to significantly improve the toughness of the base metal and welded area by appropriately selecting the chemical components of this type of low-temperature copper.

In this invention, O: o, oa ~ 0.05% by weight (hereinafter simply abbreviated as %), Si: 0.15 ~ 0.3
0%, Mn: 0.40-1.00%, Ni:
Contains a, o ~4.0%, SOI, A71:
0.01-0.04%, Ou: 0.10-0.
30%, Mo: 0.10-0.25% and cr
20.10-0.80% and total amount of Hb and ■ 0
．． 0.1% or less, with the remainder essentially consisting of iron and impurities, and after being heat-treated by normalizing, quenching, and tempering, or similar operations, it is made of a fine bainitic structure with a thick wall that has excellent strength and low-temperature toughness. It is a low temperature steel.

The reason for limiting the chemical composition of the low temperature steel in this invention is as follows.

0: 0.08 to 0.05% Conventionally, the C content of this i-steel is 0.06 to 0.10%, and even within this range, if the C content is lowered to 0%, the base metal toughness can be completely improved. By setting the content to 0.05% or less, a large width l toughness improvement effect can be seen, particularly SR embrittlement is reduced, and it is also highly effective in improving the toughness of bond parts and heat-affected zones.
The content was set at 0.05% or less, and the lower limit was set at 0.08% to ensure strength.

Si: 0-15~0.30%Si is necessary for deoxidation, so the lower limit is also regulated to prevent deterioration of the toughness of the base metal and welded part, and the upper limit is regulated to 0.16~0.30%.
The range was O, aO%.

Mn: 0.40 to 1.00% Mn 0.40% or more is required to ensure 1J strength, and 1.0
If it exceeds 0%, the toughness of the weld will deteriorate, so 0.40~
It was regulated at 1.00%.

Ni: 8.0 to 4.0% It can withstand use at -110°C and has a low C% of 0.05% or less at 50 ICgf/ll! In order to ensure a total strength of 1 or more, a minimum of 3.0% Ni is required. Upper limit of 4
．． 0% is due to economic reasons.

SOI, Al: 0.01 to 0.04% These are indispensable as a grain refining agent, and their □ effect is remarkable at o, oi% or more. If it exceeds 0.04%, the crystal grains become coarser, so it was regulated.

Ou: 0.10~0. aO% Ou increases strength without deteriorating toughness, and is added at least 0.10%. This is effective for refining carbides and reduces SR embrittlement, but 0.30
If it exceeds 7. 0.10 to 0.10 because it inhibits the
It is regulated within the range of 0.30%.

MO: 0.10 to 0.25% The only way to obtain high strength with low O% is to strengthen the structure, and by adding 0.10% or more of MO, part or all of it is converted to bainite. Furthermore, MO is a carbide-forming element, and by preventing C from diffusing into grain boundaries during SR,
Reduce R embrittlement. Addition exceeding the upper limit of 0.25% deteriorates the toughness of the bath contact area, so this regulation was made.

Or: 0.10~0. , 30% Similar to MO, at least 0.10% of S forms carbides.
It is useful for reducing R embrittlement, but if it exceeds 0.30%, the toughness of the weld zone deteriorates, so it is regulated to 0.10 to 0.80%.

Nb-4-V: 0.01% or less (9Nl) and V, a portion of which penetrates from the base metal into the weld metal and completely precipitates carbides during the next bath or IrJ during welding, and during SR after welding. Since the toughness of the weld metal will be significantly impaired, the sum of the two
It is necessary to regulate it to 0.01% or less.

The impurity that inevitably mixes into steel is mainly PIS, and although it is desirable to have both elements as low as possible, it is not economical to reduce them to an extreme level, and P IS is 0.0 by selecting other elements.
If the S content is 15% or less, S: about 0.005% or less, sufficient tenacity can be obtained.

The low-temperature steel of the present invention, which is melted with a doffing composition and then hot-rolled according to a conventional method, is heat-treated to obtain high strength and excellent toughness due to the fine bainite structure.

First, normalizing or quenching is carried out at a temperature range of 800 to 880 °0, which is necessary to develop sufficient austenite grains to obtain the required hardened structure from the austenite phase, and on the other hand to coarsen the austenite grains. This is because it is necessary to prevent the strength from deteriorating due to

Tempering is a heat treatment that imparts toughness to steel. In this case, if the tempering temperature exceeds 650℃, the tempering softens so much that the specified strength cannot be obtained, and the , the specified toughness cannot be obtained.

Note that this tempering is performed at 550 to 650°0 after the steel plate is bath-welded.
An operation similar to tempering such as SR is performed in a temperature range,
The station building is unnecessary and can be used for welding assembly as normalized or hardened.

Table 1 shows the chemical composition of the test steel according to the present invention, and Table 2 shows the mechanical properties in comparison with comparative steels.

Table 1 Chemical composition (needle stopping %) Fig. 1 also shows the relationship between the tensile strength of the base plate and the absorbed energy in the 2V Sotsushi Ruby impact test at -110°C.

The normalized comparison steels (()), (H), and (I) have low strength and toughness, whereas the strength and toughness of the test steel (B) according to the present invention are significantly improved. It is clear that there are.

As for the hardened materials, comparative steels (J) and (K) had sufficient strength, but had difficulty in ensuring toughness.

In comparison, the tenacity of the test steels (g) and (F) according to the present invention is an extremely excellent value.

Furthermore, another feature of the present invention is that the welded portion has excellent toughness. As shown in Table 2, according to the present invention, both the weld pound part and the heat affected zone have extremely superior toughness compared to conventional steel.

[Brief explanation of drawings]

Figure 1 shows the tensile strength of the base plate and 2 r at -110°C.
It is a chart showing the relationship between nm and absorbed energy in a Notch Charpy impact test.

Claims (1)

[Claims] LO: 0.08 to 0.05% by weight, Si: 0.15 to 0.80% by weight, Mn: 0.40 to 1.00% by weight, Ni: 8% by weight.
．． Included with 0-4.0% by weight, Son, AJ:
0.01-0.04% by weight. Ou: 0.10-0.80% by weight, MO: 0.
10-0.25 weight 5% and Or: o, 10-0.
The total amount of Nb and V is 0.01% by weight, which is 80% by weight.
It has excellent strength and low-temperature toughness, and is characterized by having a fine bainitic structure after being heat-treated by normalizing, quenching and tempering, or similar operations. thick-walled low-temperature steel.