CS256257B1 - Steel with increased hardenability and brittle fracture resistance in acid medium - Google Patents

Abstract

Steel with increased hardenability a resistance to acidic brittle fracture the environment is mainly intended for production steel seamless pipes used by deep well gas and oil wells a pipes used for blow molding in the mining industry. Steel with increased hardenability against brittle fracture in acidic environment, for steel production seamless tubes by hot rolling, microalloyed vanadium and niobium and dolegized boron, silicon and titanium is characterized by in that it contains in weight composition From 0.30 to 0.45% carbon; 0.90 to 1.60% manganese; From 0.30 to 0.70% silicon; 0.05 to 0.20% vanadium; 0.005 to 0.10% niobium; 0.02 to 0.06% titanium; 0.0005 to 0.002% boron; not more than 0,030% of phosphorus and not more than 0.025% sulfur, the remainder being iron and common impurities.

Description

BACKGROUND OF THE INVENTION The invention relates to the chemical composition of steel with increased hardenability against brittle fracture in an acid medium, which is particularly intended for the production of steel seamless pipes used in deep gas and oil wells. and tubes used for blow-off loading in the mining industry.

Increasing demands on the quality of oil pipes due to increasing depth of boreholes and difficult geological conditions are the reason not only for the increasing demands of pipe users to increase the basic mechanical properties by increasing the quality grades of the pipe material, but especially in recent years for use in boreholes where there is a risk of hydrogen embrittlement in particular due to hydrogen sulphide during combined tensile, compressive and bending stresses. The occurrence of stress corrosion occurs especially in deep oil and gas wells, where free hydrogen, resp. hydrogen sulphide, the amount of which varies from less than 1 ppm up to several percent by volume of the gases present in the wells. The presence of hydrogen sulphide is often the cause of premature fractures of oil pipes in wells. By adhering to certain defined technological measures in the production of pipes, in particular by selecting the chemical composition of the starting steel and its heat treatment and subsequent operations, the resistance of the pipes to hydrogen sulfide exposure can be substantially increased. The effectiveness of the measures taken can be verified both operationally by incorporating pipes into the well and by laboratory testing of steel resistance to hydrogen sulphide ·

The testing methodology chosen according to the results of the literature analysis is similar to the verification of the sensitivity of steel to embrittlement due to hydrogen sulfide and consists in immersing the tests in

- 2 *

256 257 of a 5% acetic acid solution, continuously saturated with hydrogen sulphide. The acidity of this solution is around the value

2.5 pH. During testing, samples are loaded by uniaxial tensile loading at different load levels, looking for loads that will withstand the stresses for at least 20G hours and do not crack in this corrosive environment.

The corrosion medium chosen is highly aggressive to verify the effect of hydrogen sulphide action, making it possible to distinguish and quantitatively determine the sensitivity of steel to cracking in the hydrogen sulphide environment. Until now, low- and medium-alloy steels of chromium, molybdenum or microalloyed vanadium, niobium and the like have been used to produce oil pipes for extracting oil and gas from greater depths. These steels are manufactured in electric arc furnaces and exhibit fairly satisfactory stressing properties in deep wells with increased hydrogen sulphide incidence, and also in conditions where the irrigation pH and temperature limits create conditions for accelerated corrosion processes.

These steels are used in a heat-treated state and their utility properties are due, among other things, to their hardening properties. Increasing the hardenability using nickel is unsuitable here because of the negative effect of this element on the resistance to hydrogen embrittlement. The low hardenability in the production of pipes from these materials can be solved either by the application of simultaneous external and internal hardening of the pipes or by the one-sided hardening by an improved hardenable effect of the steel used.

The above remnants are removed by vanadium- and niobium-microalloyed steel, and boron-, silicon- and titanium-doped, containing by weight 0.30-0.45% carbon; 0.90 to 1.60% manganese; 0.30 3? 0.70% silicon; 0.05 to 0.20% vanadium; 0.005 to 0.10% niobium; 0.02 to 0.06% titanium; 0.0005 to 0.002% boron; not more than 0.030% of phosphorus; not more than 0,025% of sulfur; the rest iron and the usual impurities.

The steel according to the invention, in the heat-treated state, provides a maximum resistance to hydrogen embrittlement at a high level of yield strength. Its practical use is particularly important in the oil and oil industry for the application of pipes with larger wall thicknesses. Steel can be produced by conventional steel processes in siemens-martin and tandem hearth

- 3 256 257 furnaces and electric furnaces and has a very good thermoformability due to its low alloying content. The steel is characterized by a high resistance to hydrogen embrittlement with an initial value of yield strength up to 850 MPa and at the same time a high degree of hardenability; Hydrogen brittleness resistance is furthermore ensured by the possibility of applying a minimum tempering temperature above 620 ° C for min. 720 MPa. High hardenability predetermines the steel for use in the production of blown fill in underground mines. The hardened wall of the pipe up to 20 mm deep provides considerable hardness and abrasion resistance to its inner surface even with one-sided external quenching, and this property fully replaces the costly and less hardenable double-layer pipes.

Practical examples of steel properties can be documented:

Example χ: Of steel with a content by weight of 0,41% carbon; 1.44% manganese; 0.47% silicon; 0.019% phosphorus; 0.015% sulfur; 0.07% vanadium; 0.03% titanium; 0.003% niobium and 0.0014% boron were rolled by hot rolling on Stiefel lines, heat-treated by coaxing and tempering to mechanical values: yield strength Re = 787 MPa, strength Rm »866 MPa; ductility λ = 19.9%; contractions Z = 66.9% and pipes of the same steel heat-treated to values: Re = 847 MPa;

R m = 918 MPa; Acj = 19.8% and Z = 64.5% · Both tube types exhibited a threshold stress of 650 resp. 600 MPa.

Example 2: a steel from a melting of the same chemical composition as in the first example showed a hardening depth of 18.5 mm in a standard hardenability test and, in practical application, in a pipe having an outside diameter of 194.5 mm and a wall thickness of 20 mm by quenching from 850 ° C shower achieved through hardening of 100% martensite throughout the wall thickness.

Claims (1)

Steel with increased hardenability and resistance to brittle fracture in acidic environment for the production of hot-rolled seamless steel tubes, micro-alloyed with vanadium and niobium and doped with boron, silicon and titanium, characterized in that it contains 0,30 to 0,45 by weight % carbon; 0.90 to 1.60% manganese; 0.30 to 0.70% silicon; 0.05 to 0.20% vanadium; 0.005 to 0.10% niobium; 0.02 to 0.06% titanium; 0.0005 to 0.002% boron; not more than 0.030% phosphorus and not more than 0.025% sulfur, the remainder being iron and common impurities.