JPS61124555A - Steel superior in sour resistance - Google Patents

Abstract

PURPOSE:To obtain the titled steel, by adding specified ratios of C, Si, Mn, Ca, Mg, rare earth element, Ti, Zr, Al, P, S to Fe. CONSTITUTION:Steel composed of, by weight 0.01-0.35% C, 0.02-0.5% Si, 0.1-1.8% Mn, and one or >=2 kinds among 0.0005-0.02% Ca, 0.0005-0.02% Mg, 0.001-0.02% rare earth element, 0.005-0.2% total of one or 2 kinds of Ti, Zr, further restricting to <=0.005% Al, <=0.015% P, <=0.003% S and the balance Fe with impurities is prepd. Further if necessary, one or >=2 kinds among 0.2-0.6% Cu, 0.1-1.0% Ni, 0.2-0.3% Cr, and/or one or >=2 kinds among 0.10-1.0% Mo, 0.01-0.15% Nb, 0.01-0.15% V are incorporated. By this way, the titled steel without causing hydrogen blister cracking under circumstances of low pH is obtd.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to steel with excellent sour resistance, and more specifically, it is used in steels containing wet hydrogen sulfide, for example, in oil and natural gas drilling, transportation, or storage. Concerning steel that has high cracking resistance even under environmental conditions.

(Problems to be solved by conventional technology and invention) Oil and natural gas produced in recent years often contain hydrogen sulfide, and when water such as seawater and fresh water coexists, steel In addition to thinning due to corrosion that occurs on the surface,
Hydrogen generated on the steel surface due to corrosion can penetrate into the steel, causing destruction, which has become a problem. This fracture is different from sulfide stress cracking, which has long been observed in high-strength steel, and can occur even without any external stress applied.

This fracture is caused by hydrogen penetrating from the environment that accumulates at the boundary between the steel base and F4 compound inclusions such as MnS that extend in the rolling direction in the base metal, and gasifies, resulting in gas pressure. Then, the A-based sulfide inclusions such as MnS form sharp notches, which serve as crack nuclei to grow into parallel cracks on the plate surface, and these parallel cracks are connected in the thickness direction.

This type of cracking is hereinafter referred to as "hydrogen blister cracking." Various studies have been conducted and various steels have been proposed for steels that have high resistance to hydrogen blister cracking. These include prevention of cracking by adding Cu or Co, and M
These techniques utilize reduction of nS, fixation of S by addition of Ca or rare earth elements, etc., and to date, steels that can withstand even quite severe environments have been developed using these techniques.

However, these steels have a disadvantage in that even if MnS is reduced, hydrogen bulging cracks may occur due to cluster-like large Al203 inclusions or surface inclusions. It has been found that hydrogen blistering cracking originating from large inclusions or surface inclusions cannot be prevented even by the above techniques.

Therefore, the present inventors began to consider how to prevent cracks caused by large inclusions and surface inclusions, and as a result of conducting experiments with various components, they considered reducing Al and applying Ti and Zr instead. By the way, there are few examples of conventionally using T1 for deoxidation.

Recently, a method for manufacturing steel materials with excellent toughness in the weld heat-affected zone even in high heat input welding by adding various alloying elements including T1 to generate oxides has been proposed in Japanese Patent Application Laid-Open No. 58-204.
This is reported in Publication No. 117.

]-However, this method is not originally intended to improve sour resistance, and in addition, for steels that normally have excellent sour resistance, oxygen is reduced as much as possible (150 ± 5
0) ppm, which is a very high content of 1. Therefore, it is clear that steel with excellent sour resistance cannot be obtained even by this technique.

For further reference, it has been well known that Ti improves the toughness of the weld heat affected zone, weld metal, or base metal by adding it to steel. However, when adding Ti to steel or including it in weld metal for these purposes, TiNi
The main objective is to generate TiC, and deoxidation to sufficiently reduce the amount of oxygen in the steel to prevent the formation of Ti oxides is carried out using kA as in the conventional method.

(Means for solving the problem) The present inventors have repeatedly studied steels that are free from hydrogen blistering caused by not only MnS but also large Al203-based inclusions and surface inclusions. The purpose is to reduce Al, which has been added as much as possible, to prevent the formation of large Al203-based inclusions and surface inclusions, and to replace kA with Ti.
Alternatively, by adding Zr as a deoxidizing element, it is possible to obtain a steel with significantly fewer large inclusions and surface inclusions and free from hydrogen blistering caused by large inclusions and surface inclusions, and by removing Ti or Zr. It has been found that even when added as an acid element, by limiting the amounts of P and S and adding one or more of Ca, Mg, and rare earth elements, hydrogen blistering cracking originating from MnS can be prevented. Q The present invention was made based on these findings, and its gist is that c: 0.0, 35%,
Si: 0.02~0.5%, Mn: 0.1~
1.8%, Ca: 0.0005-002%, Mg:
0.0005-002%, rare earth elements: 0.001-0
．． In addition to one or more kinds of 0.02%, one or two kinds of Ti and Zr are contained in a total of 0.006 to 0.2%,
and Az: o, oos% or less, P: 0.015% or less,
s: limited to o, ooa% or less, or further (A)
Cu: 0.2-0.6%, Ni: 0.1-1.
0%, Cr: 0.2 to 3.0% of one or more types, or (B) Mo: 0.10 to 1.0% INb
: 001-0.15 section, V: 0.01-015% 1
It is a steel with excellent sour resistance characterized by containing one or both of (A) and (B) of two or more species, with the balance consisting of Fe and impurities, especially large inclusions or kl to prevent hydrogen blistering cracks caused by surface inclusions.
The content of Al is limited to a very low 0.005% or less.
The main point is that Ti and Zr are contained as deoxidizing elements in place of oxidizing elements.

The present invention will now be described in detail.

First, the reason for limiting the range of each component in the present invention as described above will be described below.

First, C is a basic element that most stably improves the strength of steel, so it must be contained at 0.01% or more to ensure strength, but if it exceeds 0.35%, it is unfavorable for the toughness of steel. 001 to 0.3 as it has an influence.
It was set at 5%.

Next, Si is an element that improves strength, so 0.02
% or more, but the upper limit content should be 0.5% to ensure toughness.

Furthermore, since Mn is an element necessary for strength, it should be contained at 0.1% or more, but in order to ensure weldability and toughness, the upper limit content should be 1.8%. , M
g+ Rare earth elements are very effective elements for improving the sour resistance of steel materials by fixing S in steel and preventing the formation of MnS. Ca:
If the content exceeds 0.0005% or 0.02%, the effect will be saturated, so Ca: 0.0005 to 0.02%, Mg:
0.0005-0.02%, rare earth elements: 0.001
-0.02% respectively. These elements may be used alone or two or more types may be added at the same time. In addition, for the purpose of the present invention, rare earth elements have an atomic number of 57 to
Refers to one or more of each element of No. 71 and Y. In addition, T1 and Zr are the main elements used for deoxidation instead of kl, but if the total of one or two of them is less than 0006 Practically speaking, it has no deoxidizing effect and if it exceeds 0.2%, it reduces the toughness of the steel, so the content should be between 0006 and 0.2%.

In addition, as a result of detailed investigation of the cross section of steel, the present inventors found that Ti
Alternatively, when deoxidizing with Zr, the oxides containing both elements or both elements and Ca, Mg, or rare earth elements as main components are extremely fine, with most of them having a size of 1 μm or less. It was also found that there were significantly fewer large cluster-like inclusions and surface inclusions. These findings are also the reason for containing one or both of Ti and Zr in place of Al in the present invention.

On the other hand, kt should be limited to 0.005% or less because large inclusions or surface inclusions are generated as described above, and the smaller the kt, the better.

Furthermore, since P is an element that facilitates the propagation of hydrogen bulge cracks, it should be kept at 0.015% or less.

Furthermore, S combines with Mn to form MnS, which becomes the starting point for hydrogen bulging cracks, so in order to ensure sour resistance, 0.00
Must be kept below 3%.

The above is the basic component system of the present invention, but in the present invention, (A) Cu, Ni
.

One or more types of Cr or (B) one or more types of Mo, Nb, and V (-(B) one or both can be contained.

First, Cu, Ni and Cr all have the effect of improving the corrosion resistance of the base metal and reducing the amount of hydrogen penetrating into the steel.

Cu has no effect if it is less than 0.20%, and if it exceeds 0.60%, it has a negative effect on hot workability, so it should be 0.2 to 0.
．． If N1 is less than 0.1%, it will not be effective, and if it exceeds 1.0%, it may induce sulfide stress cracking, so it should be 0.1 to 10%.
% range. Note that Ni can be added at the same time as Cu within the above range for the purpose of preventing hot embrittlement caused by Cu, but even if Ni is added for this purpose, this does not deviate from the scope of the present invention. Cr has no effect if it is less than 0.02% (if it exceeds 3.0%, it reduces the toughness of the steel, so it is limited to a range of 0.2 to 3.0%.The purpose of Cr is to prevent the formation of MnS. It can also be used as an element to improve strength and toughness by adding it to steel with a Mn content of less than 0.61, and it can also be used to improve strength and toughness in other steels as well. Even if Cr is added to the steel, it does not deviate from the scope of the present invention. Next, Mo, Nb, and ■ are all elements that improve the strength of the steel, and Mo is O, ], 00%. Nb and Vl-j: The same strength improvement effect is shown by containing 0.01% or more, but Mo is 1.0%.
, Nb and V may reduce toughness if added in excess of 0.15%, so Mo should be added in an amount of 0.10 to 1.0%.
, Nb and V were limited to a range of 0.01 to 0.15%.

Each of the above-mentioned alloy components may be used alone or in combination,
Within the above-mentioned limited range, the intended effects of the present invention will not be hindered in any way.

In the steel of the present invention, if the amount of N among the impurities exceeds 0.010%, it will cause problems in weldability and is therefore undesirable. The smaller the number, the more preferable, taking into consideration the influence of the above and the toughness of the welded part. On the other hand, in order for Ca, Mg, and rare earth elements to be effectively used for fixing S, O is 0.00.
It is preferably 4% or less, and the lower the content, the more preferable it is.

As a manufacturing process for the steel of the present invention, hot rolling may be used as is.
Alternatively, it is possible to apply a manufacturing process used for ordinary steel materials, such as a controlled cooling process immediately after hot rolling, or further standardizing, tempering, or quenching and tempering the rolled material. Furthermore, a standard, tempering, or quenching and tempering process may be applied to a part or the whole of a steel pipe or container manufactured using the steel of the present invention, without departing from the scope of the present invention. Which process to apply may be determined depending on the need to ensure properties such as strength and toughness.

In addition, one of the purposes of using T1 or Zr for deoxidation in the present invention is to lower the oxygen concentration in molten steel and reduce the amount of added Ca.
, Mg or rare earth elements to effectively act on the fixation of S, deoxidation with Ti or Zr is effective for fixing Ca, Mg.
Alternatively, it is necessary to perform this before adding the rare earth element, and it is preferable to lower the oxygen concentration in the molten steel by vacuum treatment such as RH treatment after adding Ti or Zrtl.

Hereinafter, the effects of the present invention will be described in more detail with reference to Examples.

(Example) Steel having the composition shown in Table 1 was melted and hot-rolled into a steel plate with a thickness of 12 mm, followed by a thickness t4=12rrrn as shown in FIG.

A test piece 1 having a width W = 20 mm and a length t = IQO mm was taken and subjected to a sour resistance evaluation test. To investigate the influence of surface inclusions on sour resistance, test piece thickness tl
was the same as the thickness of the steel plate, and the scale from rolling was removed by polishing with emery paper.

As a sour resistance evaluation test, 5% NaCt + 0.5%
Cracks were measured by immersing them in a solution (temperature 25° C., p) (2,8-38) in which a CH,3C0OH aqueous solution was saturated with latm H2S for 96 hours. The presence or absence of cracking was determined by measuring the cracking rate by ultrasonic flaw detection in the direction perpendicular to the plate surface, and then by microscopic observation of the cross section.

The results of the evaluation test are also shown in Table 1. As is clear from Table 1, in the steel of the present invention, no cracking occurred at all, whereas in the comparative steel, cracking started from large inclusions and surface inclusions of the kt203 series. Effect) As can be seen from the above test results, the present invention provides a steel with excellent sour resistance that does not cause hydrogen blistering even in harsh environments with low pH, and is extremely valuable in contributing to the development of industry. There is something.

[Brief explanation of drawings]

Figure 1 is a diagram showing the procedure for collecting sour resistance evaluation test pieces.

Claims (4)

[Claims] (1) C: 0.01-0.35%, Si: 0.02-0.5%, Mn: 0.1-1.8% and Ca: 0.0005-0.02% in weight%. In addition to one or more of Mg: 0.0005 to 0.02% and rare earth elements: 0.001 to 0.02%, one or two of Ti and Zr are added in a total amount of 0.006 to 0.02%. 2
%, and is limited to Al: 0.005% or less, P: 0.015% or less, S: 0.003% or less, and the balance consists of Fe and impurities, and has excellent sour resistance. . (2) C: 0.01-0.35%, Si: 0.02-0.5%, Mn: 0.1-1.8%, and Ca: 0.0005-0.02% in weight%. In addition to one or more of Mg: 0.0005 to 0.02%, rare earth elements: 0.001 to 0.02%, one or two of Ti and Zr in a total of 0.00%.
6 to 0.2%, and limited to Al: 0.005% or less, P: 0.015% or less, S: 0.003% or less, and Cu: 0.2 to 0.6%, Ni : 0.1 to 1.0%, Cr: 0.2 to 3.0%.
A steel with excellent sour resistance, characterized in that the remainder consists of Fe and impurities. (3) C: 0.01-0.35%, Si: 0.02-0.5%, Mn: 0.1-1.8% and Ca: 0.0005-0.002% in weight%. In addition to one or more of Mg: 0.0005 to 0.002%, rare earth elements: 0.001 to 0.02%, one or two of Ti and Zr in a total of 0.0%.
06 to 0.2%, and limited to Al: 0.005% or less, P: 0.015% or less, S: 0.003% or less, and Mo: 0.10 to 1.0%, Nb A steel with excellent sour resistance characterized by containing one or more of the following: 0.01 to 0.15% and V: 0.01 to 0.15%, with the balance being Fe and impurities. (4) C: 0.01-0.35%, Si: 0.02-0.5%, Mn: 0.1-1.8% and Ca: 0.0005-0.002% in weight%. In addition to one or more of Mg: 0.0005 to 0.002%, rare earth elements: 0.001 to 0.02%, one or two of Ti and Zr in a total of 0.0%.
06 to 0.2%, and limited to Al: 0.005% or less, P: 0.015% or less, S: 0.003% or less, and Cu: 0.2 to 0.6%, Ni : 0.1 to 1.0%, Cr: 0.2 to 3.0%, one or more types, and Mo:
0.10 to 1.0%, Nb: 0.01 to 0.15%, V: 0.01 to 0.15%, and the remainder consists of Fe and impurities. Steel with excellent sour resistance.