US3600161A

US3600161A - Low-alloyed high strength steel having resistance to the sulfide corrosion cracking

Info

Publication number: US3600161A
Application number: US562870A
Authority: US
Inventors: Katsuya Inouye; Hideya Okada; Yuzo Hosoi; Kenichi Yukawa
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1965-07-09
Filing date: 1966-07-05
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

A LOW-ALLOYED HIGH STRENGTH STEEL HAVING A LOW SUSCEPTIBILITY TO SULFIDE CORROSION CRACKING CONTAINING

PRECENT C 0.10-0.20 SI 0.10-0.50 MN 0.20-1.00 CR 0.50-2.00 MO, LESS THAN 0.60 V, TI OR CB, LESS THAN 0.10 AL, LESS THAN 0.15 EITHER W AND/OR TA, 0.02-1.00

AND THE BALANCE BEING FE AND IMPURITIES.

Description

Aug. 17, 1971 KATSUYA INOUYE ET AL 3,60

LOW-ALLOYED HIGH STRENGTH STEEL HAVING RESISTANCE TO THE SULFIDE CORROSION CRACKING Filed July 5, 1966 I IV l/E/V 70/?5 Kafsuya [nouye H ideya O/rado Yuzo Haso/ Ken/chi Yu/rawa B);

United States Patent 3,600,161 LOW-ALLOYED HIGH STRENGTH STEEL HAVING RESISTANCE TO THE SULFIDE CORROSION CRACKING Katsuya Inouye, Hideya Okada, and Yuzo Hoso Tokyo, and Kenichi Yukawa, Kawasaki, Japan, assrgnors to Nippon Steel Corporation, Tokyo, Japan Filed July 5, 1966, Ser. No. 562,870 Claims priority, application Japan, July 9, 1965,

0/ 41,118 Int. Cl. C22c 39/14 US. Cl. 75-126F 1 Claim ABSTRACT OF THE DISCLOSURE A low-alloyed high strength steel having a low susceptibility to sulfide corrosion cracking containing Percent C 0.10-0.20 Si 0.10-0.50 Mn 0.20-1.00 Cr 0.50-2.00 Mo, less than 0.60 V, Ti or Cb, less than 0.10 Al, less than 0.15 Either W and/or Ta, 0.02-1.00

and the balance being Fe and impurities.

This invention relates to a low-alloyed high strength steel having a low susceptibility to the sulfide corrosion cracking.

In general, the addition of nickel increases most remarkably the susceptibility of steel to the sulfide corrosion cracking. For example, it is well known that the T-lA steel containing no nickel, developed by the United States Steel Co. has low susceptibility to the above-mentioned corrosion cracking as compared with the T-l steel containing Ni by the same company and this is true also about recently developed Ni free, lowalloyed, high strength steels. However, these conventional steels are yet insufiicient in regard to the susceptibility to the corrosion cracking.

Therefore, an object of this invention is to provide Ni free high strength steel having an yield strength of at least 70 kg./sq. mm. and a tensile strength of about 90 kg./sq. mm. in a quenched and tempered state and having a low susceptibility to the sulfide corrosion cracking.

Other object of this invention is to provide high strength steel having a favorable toughness at low temperatures similar to or higher than that of conventional Ni free high strength steel of an 80 kg./sq. mm. grade in tensile strength and at the same time having a low susceptibility to the sulfide corrosion cracking.

Other objects of this invention will become apparent from the following explanations referring to the accompanying drawings, in which:

FIG. 1 is a sectional view of a test piece subjected to the test for measuring the susceptibility to the corrosion cracking;

FIG. 2 is a schematic plane view of the test piece shown in FIG. 1 and FIG. 3 is an enlarged view showing a part of the test piece shown in FIG. 1.

The numerals shown in each figure indicate the dimensions of each part of the test piece and the unit is millimeter.

7 The sulfide corrosion crackingof high strength steel has been considered to be caused by hydrogen embrittlement. The inventors have confirmed, as the results of the investigations for many years about the relation between the sulfide corrosion cracking and hydrogen embrittlement, that the sulfide corrosion cracking occurs when more than a definite amount of diffusible hydrogen is absorbed in steel. The hydrogen absorbed in steel is generated by the reaction between hydrogen sulfide and steel, and hence the corrosion property of steel in hydrogen sulfide atmosphere plays an important role in the matter. Accordingly, the inventors have extensively investigated the influences of the alloying elements on the electrochemical characteristics of steel in an aqueous hydrogen sulfide and as the results thereof it has been found that the addition of a small amount of alloying elements gives almost no influences on the anodic polarization characteristics, but the cathodic polarization characteristics are considerably influenced by the kind of the alloying element to be added. Further, it has also been found that there is a correlation between the cathodic overvoltage of steel and the susceptibility to cracking, that is, if the cathodic overvoltage of steel is high and is not varied as time passes. the cracking of steel does not tend to occur. In other words, by adding the alloying elements capable of increasing the cathodic overvoltage of steel in an aqueous hydrogen sulfide, the occurrence of the sulfide corrosion cracking will be prevented. On the other hand, it has been confirmed theoretically as well as experimentally that the fracture stress of steel is higher as the crystal grain is finer.

Taking into consideration of these points, the inventors have investigated the effect of various elements on the sulfide corrosion cracking of steel, and have found that tungsten and tantalum are the effective components for satisfying the properties to the above-stated corrosion cracking of steel and that the presence of nickel gives bad influences on thus improved properties of steel by the addition of tungsten and/or tantalum.

That is, the feature of this invention is to reduce the susceptibility of steel to the sulfide corrosion cracking by adding to nickel free high strength steel tungsten and/or tantalum which is selected as an additive for satisfying the various factors as the addition elements for steel. Other feature of this invention is that molybdenum is also an element for increasing the susceptibility to cracking, but in the case of adding tungsten, the proportion of molybdenum can be reduced as low as pos-' sible, and hence by the addition of tungsten, the susceptibility to cracking can be reduced and at the same time the amount of expensive Mo can be saved.

The composition range of the high strength steel having a low susceptibility of the sulfide corrosion cracking is as follows: 0

The reason for the limitation in the composition range of the high strength steel of this invention is as follows:

The content of carbon is defined to be less than 0.2% by weight in order to increase the toughness of steel as well as to improve the weldability thereof.

Silicon is added in the amount required in the production of steel.

It is desirable to reduce the amount of manganese as low as possible in order to reduce the susceptibility to the sulfide corrosion cracking but is defined to be 0.2- 1.0%, since at least such an amount of manganese is necessary for maintaining hardenability of steel and for easy steel making.

Chormium is added to increase the hardenability of steel, and at the same time, to increase the strength as well as to improve the corrosion resistance to sulfides, but if the content of chromium is higher than 2.0%, the weldability of the steel is reduced and if less than 0.5% the above purpose will not be satisfied.

Molybdenum is required to be added less than 0.6% for increasing the hardenability, preventing the occurrence of temper embrittlement, and increasing the resistance to tempering. However, from the viewpoint of reducing the susceptibility to the sulfide corrosion cracking, the less amount is desirable, and as mentioned above, in the case of adding tungsten, the content of molybdenum may be decreased, or even omitted if desired.

Copper is an effective element for improving the corrosion resistance of steel to sulfide and to the atmospheric corrosion and the necessary amount is less than 0.5%, preferably 0.2 to 0.3%. However, Cu may be omitted as the case may be.

Vanadium contributes to increase the resistance to tempering, and titanium and columbium contribute to increase the resistance to tempering and to fine the crystal grain of the steel. They may be added alone or as a mixture thereof but for preventing the reduction of toughness of steel at low temperatures, the content of them should be less than 0.1%. Moreover, vanadium, titanium or columbium is particularly effective for the increase of tempering resistance in the case where molybdenum does not coexist or its amount is very small.

Boron is incorporated in an amount of less than 0.005% for increasing the hardenability but in the case of thin plate, boron is unnecessary.

Tungsten and/or tantalum is necessary for reducing the susceptibility to the corrosion cracking but if the content is above 1% the weldability and the toughness at low temperatures are reduced and if the content is lower than position, the steel may be subjected to conventional heat treatment, quenching and tempering. Thus, while a specific heat treatment has already been proposed to prevent the occurrence of corrosion cracking by hydrogen sulfide, but according to the present invention, the steel may be subjected to usual quenching and tempering to obtain the above-mentioned purposes. That is, the steel is heated at a temperature above the A transformation point for austenitizing, subsequently quenched, but if it is heated to a higher temperature than it needs, the crystal grain will be coarsened and the notch toughness will be reduced. Hence, the suitable heat-treating temperature is from the A transformation point to 950 C. The proper tempering temperature after quenching is from 600 to 700 C.

Further, there are no specific limitations about the furnace for smelting the steel of this invention, and any conventional furnace, such as, a converter, an open-hearth furnace, or an electric furnace may be employed.

The following examples are intended to illustrate our invention but not to limit the invention in any way.

EXAMPLE 1 The steel pieces having the chemical compositions shown in Table 1 were water-quenched from 930 C. and tempered at 600-650 C. The mechanical properties of the results are shown in Table 2.

As a process for testing the susceptibility to the sulfide corrosion cracking, there has usually been adopted an accelerative method wherein the rupture time of test piece is measured when the test piece is subjected to a definite stress or a definite strain in a 0.5% acetic acid solution saturated with hydrogen sulfide, but in order to conduct the test in the more severe conditions, the method was employed in which the rupture time of the test piece having a notch as shown in the accompanying drawing was measured while subjecting it to a definite stress corresponding to the yield stress of 90% in a 0.5% acetic acid solution which had been saturated with hydrogen sulfide. The results are shown in Table 3.

In addition, when the same test was conducted in an aqueous saturated hydrogen sulfide solution instead of employing the above-described acetic acid solution, no rupture was observed after the immersion of one week.

TABLE 1 Reference steel Steel of the present 013 013 013 013 invention 013 a. QL'JO QGCnvRCOlQP- 9999999999 Hb- Hbbh- WHWLOMPIWEQH 0 9999999999 mmmmmmmwww moooowwcewwh m E 9999999999 1mm ;-MOOM ICO OOuL'M-HQWNQQ:

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0.02%, the purpose of the addition thereof is not accomplished. The addition of tungsten and/or tantalum is the fundamental feature of the present invention and in particular, tantalum contributes to the increase of the resistance to tempering and fining of the crystal grain in steel.

Aluminum is added mainly for the purpose of deoxidation and hence the content may be less than 0.15%.

In addition, the proportions of molybdenum, vanadium, titanium, columbium, tungsten and tantalum should be determined within the above-mentioned range while considering the balance with the content of carbon, the resistance to tempering the toughness and fining of grain size.

By the reasons as mentioned above, the chemical components of the steel of this invention are limited to the above-mentioned ranges and for accomplishing the abovementioned purposes about the steel having the above com- TABLE 2 2 mm. V-notch charpy KgJmm. kg.-m./em.

Tensile strength TABLE 3 Cracking test (time) s nrated H S+0.5 CHaCOOI-I lower than that of the above-stated conventional steel. It 20 is evident from these tests that the steel of this invention has a tensile strength of above 80 kg./sq. mm. and a proper toughness as well as the steel of this invention is also excellent in susceptibility to the sulfide corrosion cracking as compared with that of conventional high strength steel of 80- kg./ sq. mm. grade.

Further, the following welding test was applied about the steel of this invention shown in Table 1 by No. 4 and No. 8.

That is, a simple layer bead was deposited on the test piece at room temperature under the conditions of 175 amperes in arc current, 25 volts in arc voltages and 152 6 mm./hr. in welding speed. The maximum Vickers hardness of the steels of No. 4 and No. 8 were 380-400, which is almost same as the result obtained about conventional high strength steel of kg./sq. mm. grade.

Further, the test samples of No. 4 and No. 8 were subjected to the modified Y-shaped slit rupture test (shown in the report Weld Cracking Test of High-Strength Steel by Hiroshi Kihara et al. in Welding Journal, vol. 41; January, 36-48, 1962), in which the test sample was pre-heated to C. or to C. and then welded under the conditions as shown above. By the result, no crack was observed in both samples.

What is claimed is:

1. A low-alloyed high strength steel having low susceptibility to sulfide corrosion crocking comprising 0.10 to 0.20% C, 0.10 to 0.50% Si, 0.20 to 1.00% Mn, 0.50 to 2.00% Cr, less than 0.60% Mo, less than 0.10% of at least one of the elements selected from the group consisting of V, Ti and Cb, less than 0.15% Al, 0.03 to 0.5% of Ta, and the balance being Fe and impurities.

References Cited UNITED STATES PATENTS 2,347,375 4/1944 Stargardter 75-126C 2,021,781 11/1935 Hildorf 75-126 2,798,805 7/1962 Hodge 75-126C 3,044,872 7/ 1967 Hayes 75126C HYLAND BIZOT, Primary Examiner US. Cl. X.R.