JPS5941486B2 - Manufacturing method of sulfide corrosion cracking resistant steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing steel materials such as steel plates, steel pipes, and steel bars that exhibit excellent sulfide corrosion cracking resistance in a humid environment containing hydrogen sulfide (H2S).

In the process of drilling, transporting, refining, and storing oil, corrosion of steel due to H2S contained in crude oil and environmental embrittlement caused by this corrosion occur frequently.

For example, sulfide corrosion cracking is always a concern in oil country pipes, line pipes, LPG tanks, etc. used in oil drilling and transportation. In recent years, steel materials used for these pipes and tanks have tended to be required to have high mechanical strength, but the problem is that the higher the mechanical strength, the more easily sulfide corrosion cracking occurs.

Furthermore, due to the strong corrosive action of H2S, steel may be corroded during use, reducing its effective wall thickness and rendering it unable to function as a steel structure. Although many studies have been conducted on steel to address this type of problem, very few low-alloy steels have been proposed that have both excellent sulfide corrosion cracking resistance and corrosion resistance.

Conventionally, the most common solution to sulfide corrosion cracking has been to set and regulate the upper limit of steel strength.

In addition, from the viewpoint of the structure of steel materials, it is said that a tempered martensitic structure that has been completely quenched and sufficiently tempered is good. For example, these ideas are adopted in the API standard 5AC regarding oil country tubular goods, which regulates the upper limit of hardness or specifies quenching and tempering as heat treatment, and further defines the lower limit of the tempering temperature. However, even if manufactured according to these standards, resistance to sulfide corrosion cracking is not necessarily guaranteed. In addition, in the case of low-alloy steel, common measures for corrosion resistance include painting, cathodic protection, or injecting inhibitors into the corrosive environment, and few measures are taken for the steel itself. The inventors of the present invention have conducted many experiments and studies with the aim of producing a steel with excellent corrosion resistance and sulfide corrosion cracking resistance. As a result, a Co-enriched layer is formed between the oxide scale and the steel matrix. This prevents hydrogen from penetrating into the steel and reduces the occurrence and propagation of sulfide corrosion cracking.
We have found that this prevents the spread of the disease.

The present invention was constructed based on this knowledge, and its gist is: C: 0.05-0.50%, Si: 0.05-1.0%.
0%, Mn: 0.10-2.00%, CO: 0.03-
A method for producing sulfide corrosion cracking resistant steel in which a steel containing 0.55% and the remainder consisting of iron and unavoidable impurities is subjected to high-temperature oxidation treatment to form a CO-enriched layer at the boundary between the oxide scale and the steel matrix. It is. The present invention will be explained in detail below.

C is added in an amount of 0.05% or more as a reinforcing component for structural materials such as steel pipes used in humid environments containing H2S.

However, an excessively contained C component increases sulfide corrosion cracking properties and lowers toughness, increasing cracking susceptibility during steel manufacturing and steel product processing, so the upper limit was set at 0.50%.
S1 is a residual component during steelmaking work, and like the C component, 0.050I) or more is required as a steel-strengthening component, and since excessive inclusion deteriorates the additivity of the steel, the upper limit is set at 1.00%. And so. Mn is a component that improves strength and toughness, but if it is less than 0.1%, this effect cannot be obtained.

Moreover, excessive content exceeding 2.0% forms significant segregation zones during agglomeration, which adversely affects toughness and sulfide corrosion cracking.

From the viewpoint of sulfide corrosion cracking resistance, the preferred range of Mn is 0.
It is 3 to 1.2%. CO in steel is 0.03-0.55
01). When CO contained in steel is oxidized at high temperatures, a CO-enriched layer is formed at the boundary between the oxide scale and the steel base due to the difference in oxygen affinity with Fe, which improves the steel's sulfide corrosion cracking resistance and corrosion resistance. Grant. Such an effect of CO cannot be obtained if the CO content is less than 0.03%, and if the content exceeds 0.55%, a saturated state is reached and the hardenability also deteriorates. Other unavoidable impurity components such as P and S should be reduced as much as possible because they deteriorate the mechanical properties, corrosion resistance, and other properties of steel. Furthermore, the effect of the CO component as described above improves mechanical properties such as strength and toughness, as well as anisotropy and weldability of steel, so Cu:
0.10-0.50%, Cr: 0.20-2.00%,
MO: 0.05-1.0001), W: 0.05-1.
00%, Nb: 0.01~0.15%, V: 0.01~
0.15%, Ti: 0.01-0.15%, B: 0.0
003-0.0050%, Ca: 0.001-0.01
00t), La+Ce: 0.001-0.050(f)
It does not disappear even in steels that selectively contain one or more of the following. The present invention manufactures molten steel having the above-mentioned composition in a melting furnace such as a converter or an electric furnace through continuous casting, ingot making, or blooming, followed by hot extrusion.
High-temperature oxidation treatment is applied to products such as steel bars and shaped steel, or products that have undergone optional post-heat treatment such as normalizing treatment.

The high-temperature oxidation treatment involves oxidizing steel at high temperature for a longer time or under higher oxygen partial pressure, and may be performed by an accelerator coating method, a steam blowing method, a low rolling ratio method, or the like. However, as shown in Figure 1, CO-containing steel that has been subjected to high-temperature oxidation treatment has a layer (CO-enriched layer) with an enriched CO concentration several times the CO content at the boundary between the oxide scale and the steel base. When formed, it acts as a corrosion protective film that prevents hydrogen generated by corrosion from penetrating into the steel in a humid environment containing H2S, and has the suppressive effect of preventing the occurrence and propagation of sulfide corrosion cracking. However, the steel products manufactured by the method of the present invention as described above have an H2
Can be used for a long period of time in a humid and corrosive environment containing S.

Next, examples of the present invention will be described.

Figure 2 shows the steel pipe sample 1 shown in the table manufactured by hot working.
The results of a sulfide corrosion cracking test when a 78 mmφ piece was subjected to high temperature oxidation treatment (heated in the air at a temperature of 800°C) are shown.

For each steel pipe, the test was carried out using a test piece with scale attached and a smooth test piece from which the scale had been removed.
Various constant stress loads were applied in a corrosive environment in which a 25° C. aqueous solution of 5% acetic acid + 5% common salt was saturated with H2S, and evaluation was performed based on the time taken to break. A and B steels that do not contain CO (
For comparison), the breaking stress of the scaled test piece is the same or slightly higher than that of the smooth test piece. However, CO
In the C and D steels (invention) containing , the breaking stress of the scaled test piece is higher than that of the smooth test piece by 10k9/M77l or more, and is also higher than that of the A and B steels.

[Brief explanation of drawings]

Figure 1 shows CO-containing steel subjected to high-temperature oxidation treatment (temperature 750℃).
Figures 2A, b, c, and d show the results of measuring the CO concentration on the steel surface with an electron probe microanalyzer when the steel surface was heated (heated in the atmosphere). FIG.

Claims (1)

[Claims] 1C: 0.05-0.50%, Si: 0.05-1.
00%, Mo: 0.10-2.00%, Co: 0.03
A sulfide-resistant product characterized by forming a Co-enriched layer at the boundary between the oxide scale and the steel base by subjecting steel containing up to 0.55% and the remainder consisting of iron and unavoidable impurities to high-temperature oxidation treatment. Method of manufacturing corrosion-crackable steel.