JPS6012411B2 - Manufacturing method for corrosion-resistant steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a steel material that exhibits excellent corrosion resistance to both seawater and hydrogen sulfide and has good mechanical properties.

With the tightening of petroleum resources, mining of offshore oil and gas fields is becoming more active, but since the mining pit is located on the ocean floor, the materials for wellhead equipment are more expensive than those derived from onshore oil or gas fields. Strict mechanical properties and corrosion resistance are required.

In other words, excessive water pressure is applied to the wellhead equipment, and it is almost impossible to replace it, so it must be of high strength (for example, 100k9f/side 2 or more) to ensure a sufficient safety factor, and must be resistant to sudden destruction. Not only does it have extremely high plowability (e.g. 6K9 one skin/side 2 or more), but it also has excellent corrosion resistance against corrosive gases in submarine oil (especially hydrogen sulfide) and seawater itself, and does not cause pitting or crevice corrosion. , and low sensitivity to cracking. Possible metal materials that can satisfy these requirements include Ti, Ni--Co alloys, and Ni-based alloys, but they are extremely expensive, which poses the problem of pushing up the supply price of submarine oil and submarine natural gas. On the other hand, the present inventors have focused on semi-austenitic precipitation hardening stainless steel and have also conducted extensive studies on the relationship between heat treatment conditions, mechanical properties, and corrosion resistance. As a result, it was found that the conventional heat treatment process had various problems. That is, as a precipitation hardening stainless steel, for example, AM355 (AI
S1634) is known, but various recommended heat treatment methods (for example, sub-zero incineration followed by tempering, double aging, double transformation treatment, cold rolling followed by low-temperature tempering, etc.) have been established for this material. Excellent mechanical properties (tensile strength: 120-250 k9f/side 2) can be obtained.

However, many of the materials obtained through the above heat treatment generally exhibit sulfide stress cracking susceptibility, and cannot be fully recommended as materials for submarine wellhead equipment, and the heat treatment process is complicated and economically problematic. Ta. The present invention has been made in view of these circumstances, and has been developed to provide a steel material that not only has high strength and toughness, but also exhibits excellent corrosion resistance against seawater as well as against hydrogen sulfide. As a result of various studies on a method that can reliably and inexpensively produce the following using a versatile heat treatment method, the present invention has been completed. The present invention, which has achieved the above object, has C: 0.05 to 0.15% (weight %, same hereinafter) Si
: 0.50% or less Mn: 0.5-1.5% Cr: 14.0-17.0% Ni: 3.0-6.0% Mo: 2.0-3.5% V: 0. 05-0.15% N: 0.05-0.18% Fe and unavoidable impurities: The steel material consisting of the remainder was solution treated at 1000-110° and then rapidly cooled, and then tempered at 250-5500°. , is a highly versatile heat treatment method that not only supplements a certain amount of V to the chemical composition of AM355 but also performs low-temperature tempering after solution treatment. By adopting this method, it has become possible to reliably and inexpensively provide steel materials with high strength, high quality, corrosion resistance, and cracking resistance.

The reasons for limiting the chemical components and heat treatment conditions will be explained below.

'1} Chemical component C: 0.05-0.15% C is an essential element for improving strength, and at least 0.
．． However, if it exceeds 0.15%, the corrosion resistance decreases, so 0.15% is set as the upper limit.

Si: 0.50% or less Sj is an essential component for appropriate deoxidation and purification of steel, but
Excess addition actually impairs the cleanliness of the steel, so the upper limit should be set at 0.
It was set at 5%.

Mn: 0.5-1.5% Like Si, it is essential for deoxidizing and purifying steel, but at the same time it is
It is an essential component for stabilizing stenite and increasing the strength of steel.

For this purpose, it is necessary to add 0.5 or more, but since excessive addition causes problems such as inhibiting hot workability,
．． The upper limit was set at 5%. Cr: 14.0-17.0% An essential element for improving corrosion resistance. If it is less than 14.0%, corrosion resistance against seawater will be insufficient. However, Cr is originally a ferrite-forming element, and if added in excess, The upper limit was set at 17.0% because 6-ferrite is likely to be formed during heating, causing problems in sulfide stress cracking resistance. Ni: 3.
0 to 6.0% Ni, like Cr, is essential for improving corrosion resistance and is an element necessary for stabilizing austenate, and stable austenate cannot be obtained at less than 3.0% Ni.

However, since it is an expensive element, excessive addition would reduce the advantage of cost reduction, so the upper limit was set at 6.0%. Mo: 2.0 to 3.5% Mo is essential for improving corrosion resistance and must be contained at 2.0% or more.

However, it is a ferrite-forming element similar to Cr, and 3.5%
Exceeding this is not preferable because it makes it easier to form 6-ferrite and reduces cost benefits. V:0.05~0.
15% This is an element that is not normally included in the aforementioned precipitation hardening stainless steel (AM35 groove, etc.), but when added to it,
It has the ability to improve each item of strength, grade, and corrosion resistance, and in order to reliably exhibit these functions, it is necessary to contain 0.05% or more.

However, if it exceeds 0.15%, there arises the disadvantage that the corrosion resistance and strength are reduced at the same time. N
: 0.05-0.18% It is an important element for the formation of austenite, and indirectly contributes to improving corrosion resistance and yield strength, but in order to exhibit this effect, a content of 0.05% or more is required. It is.

However, if it exceeds 0.18%, there will be many flow holes during casting, causing problems such as inhibiting hot workability.
'2} Heat treatment conditions Solution treatment temperature: 1000 to 110000 Solution treatment is performed for the purpose of interdissolving Cr carbide, softening by recrystallization, improving corrosion resistance, removing internal stress, etc. If the temperature is lower than that, the solid solution of the alloying elements will be insufficient and undissolved carbides will be produced, making it impossible to achieve the intended purpose.

However, heating in excess of 110 and 0 causes coarsening of crystal grains and precipitation of 6-ferrite, making it impossible to maintain an austenite structure and causing deterioration of corrosion resistance. A more preferable temperature range is 1020 to 1080q○, and an even more preferable temperature range is 1
040 to 1060 times 0. The heating time is increased in proportion to the thickness of the steel material, but generally it may be set at a rate of 1 hour per 25 side thicknesses. Regarding cooling of steel materials that have undergone solution heating, for example, there is a problem that Cr carbides are likely to precipitate at grain boundaries in the region of 900 to 500°C, so this region is particularly rapidly cooled, thereby ensuring excellent corrosion resistance. It is necessary to. For example, if the steel is left to cool after the solubilization treatment, the Cr carbide will precipitate, the austenite will become unstable, martensitic transformation will occur, and corrosion resistance, particularly sulfide cracking resistance, will deteriorate significantly. Tempering: 250~
5500○ This step is carried out for the purpose of alleviating the hardening effect caused by the rapid cooling, and aims to eliminate the thermal strain caused by the solidification treatment and to improve the sardine properties.

In this sense, tempering at a temperature of 25,000 or higher is desired. At temperatures below 250°C, thermal strain cannot be removed sufficiently.
There is a high risk of stress corrosion cracking during use.

On the other hand, if it exceeds 550 pm, grain boundary precipitation of carbides occurs, which deteriorates corrosion resistance, which is disadvantageous. A more preferable temperature range is 350 to 5000°, and an even more preferable temperature range is 400 to 500°.
It is 00q○. Since the chemical composition and heat treatment conditions of the steel of the present invention are as described above, the strength, whipping properties, corrosion resistance, etc. are definitely improved, and since it is particularly susceptible to corrosion caused by seawater and hydrogen sulfide, it is suitable for use in submarine oil fields and submarine gas. It is the best material for equipment used in corrosive environments such as rice fields.

Next, examples of the present invention will be described.

Example 1 A steel material having the following composition was subjected to solution heat treatment (1 hour) at 1040 oo, then quenched by immersion in an oil bath, and further heated for 1 hour.
Select one temperature condition from 50 to 75,000 and set 3
Tempering was carried out for an hour and then air cooled.

C: 0.12% Si: 0.25
%Mn: 0.90% Ni: 4.5
0% Cr: 15.60% Mo:
2.90%V:009%N:
When the tensile strength, elongation, and V-notch Charpy impact value were measured for steel material A with 0.150% solid solution and steel material B with various rescaling temperatures, the results were shown in Figure 1. When the influence of tempering temperature on various corrosion resistance properties was investigated, the results shown in Table 1 were obtained.

The test and evaluation methods for various corrosion resistance performances shown in Table 1 are as follows. Sulfide stress cracking test> The test piece was placed in artificial seawater containing 2S (at room temperature) at 11.5, 20.0, 32.5 kgf/side 2.
After being immersed for 500 hours while applying stress, the presence or absence of cracks was observed and evaluated on the following four levels.

◎: No cracking sensitivity under stress of 32.5k9f/side 2.
:20,. k9f/side 2″△:11,5k9
f/fence 2 〃×: 11.5k9f/susceptible to cracking under stress on side 2 * In addition, the above ◎~× correspond to ◎~× listed below in Table 1 (the same applies below) <chloride cracking resistance Properties > The test piece was immersed in air-saturated artificial seawater maintained at 100 qo with stress applied by bending it into a V shape for one month, and then the presence or absence of cracks was observed and the erosion rate was calculated. , evaluated in the following four stages.

◎: No cracking is observed, and the corrosion rate is almost 00: No cracking is observed, but some corrosion weight loss is observed. △:
Although no cracks are observed, there are signs of cracking (ex pitting corrosion).

×: Cracking was observed.

<Crevice corrosion resistance> Two test pieces sandwiched with a gap between 50 and 7
000, immersed in artificial seawater maintained at pH 3.5 for 1 to 2 weeks, descaled, measured the maximum crevice corrosion depth with a dial gauge, and calculated the erosion rate from the weight loss. It was evaluated in stages.

◎: Neither crevice corrosion nor corrosion weight loss was observed.

○: No crevice corrosion observed.

△: Signs of crevice corrosion (discoloration) are observed.

×: Crevice corrosion occurred. <Pitting Corrosion Resistance> A test piece was immersed in air-saturated artificial seawater at room temperature, and a potential of 1,100 to 300 hV was applied, and the test piece was evaluated in the following four stages based on the length of pitting corrosion occurrence time.

◎: No pitting corrosion occurs within 1 hour when polarized at +30hV 0
: 130 blood 〃 Yes *△: +10
Enemy hV 〃 Yes ×: 110th place h
V 〃 Pitting corrosion occurred within 10 minutes Table 1 ◎: Very good, 〇: Good. △: Slightly bad, ×: Very bad As shown in Figure 1, the tensile strength was 65.
000 or 75000, the higher the temperature, the higher the tendency to improve, but the V-notch Charpy impact value was 6.
The elongation decreased sharply with 5030 tempering, and the elongation showed a decreasing tendency with 750qC tempering.

As far as these physical properties are concerned, there was no inconvenience at all even when the solid solution solution was used, but as shown in Table 1, material A that was not tempered had poor sulfide cracking resistance and chloride cracking resistance. The results were somewhat poor. These cracking resistances showed a tendency to improve as the tempering temperature became higher, but they deteriorated again when the tempering temperature exceeded 65,000.
In addition, in B, there remained a problem in sulfide cracking resistance, and in F, problems occurred in crevice corrosion resistance and pitting corrosion resistance, so the suitable temperature range for tempering is over 15,000 qo and 550 qo.
It was found that less than

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between tempering temperature and physical properties of steel. Figure 1

Claims (1)

[Claims] 1 C: 0.05 to 0.15% (weight %, same hereinafter) S
i: 0.50% or less Mn: 0.5-1.5% Cr: 14.0-17.0% Ni: 3.0-6.0% Mo: 2.0-3.5% V: 0 .05-0.15% N: 0.05-0.18% Fe and unavoidable impurities: The steel material consisting of the remainder is solution treated at 1000-1100°C, then rapidly cooled, and then tempered at 250-550°C. ,
A method for producing a corrosion-resistant steel material, which is characterized by obtaining a steel material having an austenitic structure.