SK283761B6 - Steel for heat-resistant and/or high-tensile formed parts - Google Patents

Abstract

The invention relates to a steel for heat-resistant and/or high-tensile strength formed parts, in particular, pipes, metal plates and forged pieces, with an increased heat-resistance up to 600 °C and increased plasticity and impact strength up to -60 °C and with the following chemical composition: carbon C = between 0.02 and 0.3 %, silicon Si = between 0.0 and 0.8 %, manganese Mn = between 0.0 and 2.0 %, chrome Cr = between 0.0 and 3.5 %, molybdenum Mo = between 0.0 and 1.5 %, vanadium V = between 0.02 and 0.8 %, niobium Nb = between 0.0 and 0.1 %, nickel = between 0.0 and 2.5 %, titanium Ti = between 0.0 and 0.1 %, phosphorous P = max. 0.05 %, sulphur S = max. 0.05 %, aluminium Al'total' = between 0.0 and 0.05 %, copper Cu = between 0.0 and 0.8, the remainder made up of iron Fe and nitrogen N. The inventive steel is characterised in that the total content of nitrogen N is determined by the following relation: N = (0.52 multiplied by Al + 0.29 multiplied by Ti + 0.075 multiplied by Nb + 0.005 to 0.07) %.

Description

Technical field

The invention relates to steels for the manufacture of refractory parts with increased plasticity, operating in highly stressed engineering, power and chemical plants at temperatures from + 450 ° C to 600 ° C, for example tubes, sheets and forgings.

BACKGROUND OF THE INVENTION

Up to now, heat-resistant molded parts, such as boiler tubes operating at elevated temperatures up to 600 ° C, are made from either low-alloy chromium-molybdenum (CrMoV) steels or chromium-molybdenum (CrMo) steels.

The CrMoV steels used have the following chemical composition by weight: C = 0.08 to 0.30%, Mn = 0.4 to 0.7%, Si = 0.15 to 0.4%, Cr = 0.3 to 1, 7%, Mo = 0.4 to 1.2%,

V = 0.22 to 0.7%, Al _ce ik = max. 0.04%, P = max. 0.04%, S = max. 0.04%, the remainder being Fe and unavoidable manufacturing impurities. The production impurities also included nitrogen, the amount of which ranged from N = 0.004 to 0.013%, depending on the melting aggregate used in the production of steel, but this was not exactly specified.

In the manufacture of the product, e.g. Tubes, made of this steel, are hot or cold formed and heat treated. The heat treatment consists of normalization annealing and tempering or quenching and tempering. Heating to normalization or quenching temperature, which is about 900 to 1000 ° C, dissolves the carbides, but should not completely dissolve the carbides and thus the grain coarsening. The desirable precipitation of the vanadium carbide dispersion phase occurs, in particular, in the tempering of the steel, which is carried out at temperatures of 650 to 740 ° C.

In practice, it has been found that even with the required chemical composition, including micro-purity and the usual heat treatment method, a fine-grain structure is produced for products from some melts and a coarse-grain structure is observed for products from other melts, negatively affecting the strength and plastic properties detectable immediately in acceptance tests. Products from such melts, where a coarse-grained structure is formed, are then unusable, which significantly worsens the economy of production. It is also possible to observe the dimensional instability of the precipitated carbides in products from some melts, which in particular negatively affects the creep resistance. This property is detectable by long-term creep tests or by prolonged exposure of the products at elevated temperatures, but not detectable in acceptance tests.

U.S. Pat. U.S. Patent No. 5,876,521 discloses a high-strength steel with a fine-grained martensite-bainite structure having the following chemical composition by weight: C = 0.03 to 0.12%, Si = 0.10 to 0.50%, Mn = 0.40 to 2.0%, Cu = 0.50 to 2.0%, Ni = 0.50 to 2.0%, Nb = 0.03 to 0.12%,

V = 0.03 to 0.15%, Mo = 0.20 to 0.80%, Cr = 0.30 to 1.0%, Ti = 0.005 to 0.03%, Al = 0.01 to 0, 05%, N = 0.001 to 0.01%. The fine-grained structure of vanadium carbide in this steel, which contains only a very small amount of carbonitride, is achieved by thermo-mechanical processing under stringent temperature and reducing conditions. Since vanadium carbides dissolve at high temperatures and thereby cause grain growth, this steel is unsuitable for use in the working temperature range of 500 ° C to 600 ° C.

From the Japanese abstract vol. 1997, no. 11, of November 28, 1997, JP09 184043A, a low-alloy steel with the following chemical composition by weight is known: C = 0.05 to 0.25%, Si = 0.1 to 1.0%, Mn = 0.2 to 2.0%, Cu = 0.01-0.5%, Ni = 0.01-1.5%, Cr = 0.01-0.7%, Mo = 0.01-0.7%, V = 0.001 to 0.07%, N = 0.002 to 0.01%, Al = 0.01 to 0.025%, Nb = 0.001 to 0.05%, Ti = = 0.001 to 0.025%, with N = (0, 4 x Al + 0.15 x Nb + + 0.3 x Ti + 0.0005 to 0.005)%, the rest being iron and unavoidable manufacturing impurities. Wrought parts made of this steel are unsuitable for use in the temperature range from 450 ° C to 600 ° C due to the low vanadium and nitrogen contents and hence the low vanadium carbide content.

During thermomechanical or normalization rolling, respectively. during the heat treatment by normalizing the products of these steels, the temperature and time of the heat treatment as well as the cooling rate after thermo-mechanical or normalization rolling, respectively. after heat treatment, which are decisive for the resulting strength and plastic properties of the final product. Rapid controlled cooling results in the precipitation of fine, dimensionally stable and evenly distributed vanadium carbides and / or niobium carbides, which structure is absolutely necessary for the desired strength and plastic properties. Failure to observe the cooling rate and / or heat treatment time during heat treatment will result in the formation of undesirable coarse, dimensionally unstable and unevenly distributed vanadium carbide and / or niobium precipitates as a result of the high carbide roughing rate and / or high carbide dissolution rate; and / or to form a coarse-grained structure. A product with this structure is then unusable as it does not meet the required strength and plastic properties. The disadvantage of this steel is therefore its high production technology, including heat treatment.

SUMMARY OF THE INVENTION

The above disadvantages are essentially eliminated by the steel for the refractory molded parts for use in the operating temperature range of 450 ° C to 600 ° C with the following chemical chemical composition: C = 0.05 to 0.3%, Mn = 0.01 to 1, 2%, Si = up to 0.8%, Cr = 0.3 to 3.5%, Mo = 0.2 to 1.5%, Ti = up to 0.02%, V = 0.23 to 0.8 %, Al _total = up to 0.05%, P = up to 0.05%, S = up to 0.05%, N = (0.52 x Al _cc content + + 0.29 x Ti content + 0.005 to 0 , 07)%, the remainder being Fe and unavoidable manufacturing impurities.

Wrought products of such steel, after normalization annealing and tempering, or after quenching and tempering, always have a uniformly fine-grained structure with vanadium carbonitrides and, consequently, high strength, respectively. heat resistance and plasticity at elevated temperatures. There is therefore no longer any economic loss and production of non-gifts caused by the previously unwarranted product quality. The guaranteed fineness and uniformity of the grain is achieved by the fact that normalization and tempering or quenching and tempering do not completely dissolve the vanadium precipitates causing the grain growth, since vanadium forms more difficultly soluble vanadium carbonitrides with nitrogen. Vanadium carbide would dissolve completely under the same conditions and grain would coarsen. The rate of vanadium carbonitride roughening is lower than the rate of vanadium carbide roughening, so that the vanadium carbonitride dispersion phase precipitates in finer and dimensionally more stable form. As a result, the heat-resistant properties are also improved. Thus, it can reasonably be expected that the creep rupture strength of steel containing vanadium carbide nitride will be higher than that of vanadium carbide-reinforced steel.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

For the production of a highly stressed tube intended for operation at temperatures up to 600 ° C, steel according to ČSN 41 5128 with the following chemical chemical composition was used: C = 0.12%, Mn = 0.56%, Si = 0.58%, Cr = 0.64%, Mo = 0.43%, V = 0.25%. Al _ce | _k = 0.031%, P = 0.008%, S = 0.009%, N = 0.0116%, Cu = 0.09%, Ti = 0.02%, Ni = 0.12%, the remainder being Fe and necessary manufacturing impurities.

A tube with dimensions of 0 273 x 20 mm was hot rolled and heat treated in a conventional manner, i. j. normalizing (960 ° C / air) and tempering (720 ° C / air). After this heat treatment, the tube had non-compliant mechanical properties listed in Table I on line 1, since the nitrogen content of the steel N = 0.0116% was lower than the condition of the invention, namely:

N = (0.52 x 0.031 + 0.29 x 0.02 + 0.05 to 0.07)% = = 0.0269 to 0.0919%. A pipe from such a melt is unusable for this purpose.

Example 2

For the production of the same tube, steel according to ČSN 41 5128 with the following chemical chemical composition was used: C = 0.13%, Mn = 0.54%, Si = 0.27%, P = 0.018%, S = 0.015%, Cr = 0.73%, Mo = 0.46%, V = 0.25%. Alceic = 0.005%, Cu = 0.08%, Ti = 0.002%, Ni = 0.13%, N = 0.0132%.

A tube with dimensions of 0 273 x 20 mm, made of this steel, after quite the same treatment as in Example 1, had satisfactory mechanical properties as shown in Table 1, line 2, since the nitrogen content N = 0.0132% complies with the inventive nitrogen content condition:

N> (0.52 x 0.05 + 0.29 x 0.002 + 0.005 to 0.07)% = = 0.0082 to 0.0432%.

The mechanical properties and notch toughness of this steel tube fully meet the required values.

cell resulting in the described non-compliant parameters.

However, if the amount of nitrogen present in the steel is a condition of the invention, the steel does not contain vanadium carbides, but only vanadium carbonitrides, which do not dissolve in said heat treatment, thereby preventing grain from coarsening.

Industrial usability

The steel according to the present invention is suitable for the production of molded parts which are required to be of higher refractoriness and high strength, while at the same time increasing plasticity in the working temperature range of 450 ° C to 600 ° C, which are used in particular on highly stressed engineering, power and chemical plants , for example. boiler tubes, sheets, forgings.

Claims (1)

PATENT CLAIMS 1. Steel for a refractory molded part, in particular for use in a temperature range of 450 ° C to 600 ° C, characterized in that its mass chemical composition is: carbon C = 0.05 to 0.3%, manganese Mn = = 0.01 to 1.2%, silicon Si = up to 0.8%, chromium Cr = 0.3 to 3.5%, molybdenum Mo = 0.2 to 1.5%, vanadium V = 0.23 to 0 8%, titanium Ti = up to 0.02%, aluminum Al _cdk = up to 0.05%, phosphorus P = up to 0.05%, sulfur S = up to 0.05%, nitrogen N = (0.52 x Al + + 0.29 x Ti + 0.005 to 0.07)%, the remainder being iron Fe and unavoidable manufacturing impurities. End of document Table I Chemical composition Mechanical properties wt% Rp02 rm KCV pr. C Cr Mo IN you Al _c dk N mpa J / ctn ² 1 0.12 0.64 0.43 0.25 0.02 031 0.0116 446 591 20 2 0.13 0.73 0.46 0.25 0,002 0,005 0.0132 460 601 213 Where Rpoj is the shear limit at 20 ° C, R _m is the strength limit at 20 ° C, KCV is the notch toughness at 20 ° C As shown in Table I, the modified nitrogen steel of Example 2 has significantly better mechanical properties than the non-modified steel of Example 1. The reason for these striking differences is the different grain structure and grain size between the steels of Examples 1 and 2. In the steel of Example 1, vanadium is present in the form of vanadium carbides, which dissolve more rapidly than carbonitrides and cause thermal treatment of the product, i. j. during normalization or quenching and tempering or thermo-mechanical forming at temperatures above Ac3, grain growth and therefore a coarse-grained, uneven o-