JPS5940889B2 - Manufacturing method for steel materials with excellent resistance to hydrogen-induced cracking - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a steel material having a strength equivalent to API standards X-42 to X-80 and excellent resistance to hydrogen-induced cracking.

Pipeline construction has become popular in recent years, but embrittlement of materials due to corrosion has become a problem in these pipelines.

In particular, in the pipeline transportation of oil and natural gas,
Crude oil and natural gas often contain hydrogen sulfide, and there is a high probability that these hydrogen sulfides coexist with water and contribute to corrosion of the steel surface, and in this case, the hydrogen generated by corrosion penetrates into the steel. Destruction is a problem.

This fracture due to hydrogen penetration is a different phenomenon from stress corrosion cracking caused by general sulfides. Cracks occur inside the steel under no stress load, and in the case of steel plates, cracks occur in a straight or step-like manner within the cross section of the plate. It has a continuously connected form.

If this crack becomes a through crack that extends to the front and back surfaces of the board,
This type of cracking causes oil leakage and leads to destruction of the pipeline, and in this invention, such cracking is referred to as hydrogen-induced cracking.

Hydrogen-induced cracking occurs when a steel plate made from a material with internal segregation, such as Qζ continuous casting material or a large steel ingot with a thickness of 400 mm or more, is used for a line pipe. /4 (where t is the plate thickness) or more, due to the micro-segregation of manganese, phosphorus, etc., a low-temperature transformation structure such as martensite is generated, and hydrogen-induced cracking occurs along this structure, which causes pipeline damage. Destruction accidents are likely to occur.

The present invention proposes a method for manufacturing a steel material that has the toughness to withstand embrittlement under such harsh environments and is resistant to hydrogen-induced cracking.

This invention is produced by continuous casting or from a large steel ingot with a thickness of 400 mm or more, containing 0.05 to 0.15% carbon, 0.5% or less silicon, 0.9 to 1.6% manganese, and phosphorus. 0.
0.03% or less, sulfur 0.002 to 0.015%, copper 0.03% or less
20 to 0.40%, aluminum 0.1% or less, further niobium 0.1% or less, vanadium 0.1% or less, molybdenum 1% or less, chromium 0.5% or less, nickel 0.3% or less After hot rolling a steel billet containing one or two of
This is a method for producing steel materials with excellent hydrogen-induced cracking resistance and a strength equivalent to API standards X42 to X80 by cooling steel pieces at a cooling rate of 50 to 1000°C/hr.

As mentioned above, the main purpose of this invention is to prevent hydrogen-induced cracking by preventing the formation of low-temperature transformed structures such as martensite that occur when cooling after hot rolling. The phenomenon of cracking occurs in continuous casting materials where low-temperature transformed structures based on the above-mentioned microsegregation are prominent.
The above-mentioned steels manufactured from large steel ingots with a thickness of 400 mm or more are particularly notable.

Therefore, the present invention is directed to steel materials having the above-mentioned components, which are produced by continuous casting and from large-sized steel ingots having a thickness of 400 mm or more.

If the carbon content in the target steel of this invention is less than 0.05%, the required strength cannot be obtained, and if it exceeds 0.15%, weldability will be impaired, so the carbon content is set at 0.05 to 0.15%.

Silicon is necessary as a deoxidizing agent, but if it exceeds 0.5%, it increases brittleness and is therefore undesirable.

Manganese is necessary for increasing toughness and as a desulfurizing agent,
If it is less than 0.9%, sufficient toughness cannot be obtained, and if it exceeds 1.6%, the toughness will deteriorate.

Tsunba is an element that forms a low-temperature transformation structure, and has a great relationship with hydrogen-induced cracking, so the lower the content, the better.
．． 0.3% or less.

Iori has a sensitive effect on hydrogen-induced cracking and has a 0.0
], exceeding 5% is undesirable because cracking susceptibility increases.

Moreover, if it is less than 0.002%, manufacturing becomes difficult.

Copper is effective in improving strength and reducing hydrogen absorption, but 0.
If it is less than 2%, the effect cannot be expected, and if it is added in excess of 0.4%, the effect will not improve much.

Aluminum is necessary as a deoxidizer for steel, but 0.1
% is undesirable because the toughness deteriorates.

Niobium, vanadium and molybdenum are effective in improving strength and toughness, but niobium, vanadium
．． If molybdenum exceeds 1%, the effect will be almost saturated and the cost of potash will rise, which is not preferable.

Chromium is effective in improving strength and reducing hydrogen absorption, but if it is added in an amount exceeding 0.5%, the effect reaches a saturated state and is also economically disadvantageous.

Nickel is generally an element effective in improving toughness, but in this invention, the purpose is to improve hot workability and weldability when copper is added, but if the copper content is 0.4% or less, The purpose can be sufficiently achieved with 0.3% or less.

Furthermore, after hot rolling, control of the cooling process is most important and slow cooling is necessary in order to prevent the formation of low-temperature transformed structures such as martensite in the central part of the steel billet where there is micro-segregation.

However, slow cooling at temperatures above 650°C is undesirable because it impairs the strength and toughness of the steel, and slow cooling at temperatures below 250°C is meaningless because the low-temperature transformation has finished.

Therefore, in this invention, the temperature during the cooling process is 65
It is regulated that steel slabs at 0 to 250°C are slowly cooled.

The reason why the upper limit of the cooling rate in the temperature range of 650 to 250°C is set to 1000°C/hr is that if the cooling rate exceeds this, it becomes difficult to prevent the formation of a low-temperature transformed structure.

Furthermore, cooling slower than 50° C./hr is not preferable because it causes deterioration in toughness such as tempering brittleness.

Further, the hydrogen-induced cracking resistance of the steel material according to the present invention remains unchanged even after tempering, and has excellent properties.

Next, examples of the present invention will be described.

Tables 1 and 2 compare test results regarding mechanical properties and hydrogen sulfide cracking of steel plates produced with a thickness of 12 mm by the production method according to the present invention and the conventional production method. It is.

The first hydrogen sulfide cracking test listed in Table 2 was conducted as follows.

That is, as shown in Figure 1, a test piece with a width (W') of 25 mm, a thickness of t) of 9 mvt, and a length of 1. After being immersed in a solution saturated with hydrogen sulfide for 96 hours without stress loading,
The cross section was examined and hydrogen-induced cracking was determined.

In this test, 20 test pieces were prepared for materials having the component compositions A to ①, and 10 test pieces were used for each cooling rate.

As is clear from Table 1.2, when the present invention is implemented,
The mechanical properties are the same as before, and regarding hydrogen sulfide cracking,
Although there were no occurrences of step-like cracks or linear cracks, it is recognized that the above-mentioned cracks are likely to occur if either or both of the component composition and cooling rate do not fall within the technical scope of the present invention. .

Therefore, it can be seen that the present invention is extremely effective in preventing the above-mentioned hydrogen-induced cracking.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the sampling position and dimensions of a test piece in a hydrogen sulfide cracking test.

Claims (1)

[Scope of Claims] 1. A steel material manufactured from a continuous cast material or a large steel ingot with a thickness of 400 mmJJ, which contains carbon 0.05 to 0.1
5%, silicon 0.5% or less, manganese 0.9-1.6%
, Phosphorus 0.03% or less, Sulfur 0.002-0.015
%, copper 0.20-0.40%, aluminum 0.1% or less, niobium 0.1% or less, vanadium 0
．． 1% or less, molybdenum 1% or less, chromium 0.5% or less, nickel 0.3% or less, and the remainder is essentially iron. A method for producing a steel material with excellent resistance to hydrogen-induced cracking, characterized by cooling a hot water temperature range of ~250°C to 50°C to 50°C.