HIGH-STRENGTH HEAT-RESISTING FERRITIC STEEL PIPE AND TUBE

Abstract: A high-strength, heat-resisting ferritic steel pipe or tube for boiler use containing 0.03 to 0.15% C, 0.1 to 1.5% Mn, 8 to 13% Cr, more than 1.5% up to 3.0% W, 0.05 to 0.30% V, 0.02 to 0.12% Nb, 0.02 to 0.05% N, up to 1.0% Mo, and up to 0.25% Si. This pipe or tube has an improved high-temperature creep rupture strength and an excellent weldability and toughness.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-­ strength heat-resisting ferritic steel pipe or tube, more particularly, to a heat-resisting ferritic steel pipe or tube containing chromium, the pipe or tube having improved high temperature creep characteristics and excellent weldability and toughness. 2. Description of the Related Art In recent years, in the field of thermal power plant, plant sizes are increasing and the operating temperatures and pressures are rising. When selecting steel pipe or tube (hereinafter in this section, both collectively referred to as steel tube) for use at elevated temperatures exceeding 550°C, inevitably higher grade austenitic steel tubes, such as 18-8 stainless steel tubes are used instead of 2¼ Cr-1Mo ferritic steel tubes, from the viewpoint of oxidation resistance and high temperature strength. As the grade of steel used becomes higher, i.e., from low alloy steel tube to stainless steel tube, or further, to super alloy tube, both tube and boiler construction costs are increased. This has led to the use of a super critical pressure boiler having an increased operating pressure, to improve boiler effi­ ciency. A steel tube that will fill the gap between 2¼ Cr-1Mo steel tubes and austenitic stainless steel tubes has been desired for many years. However, steel tubes with intermediate contents of Cr, i.e., 9Cr, 12Cr, etc., have an impaired weldability corresponding to an increase in the strength as compared with 2¼ Cr-1Mo steel tube. These steel tubes cannot be practically used because the impaired weldability considerably lowers the efficiency of boiler fabrication work. Under these circumstances, research has been made by the present inventors and others into the development of novel steel tubes having an improved weldability and a creep rupture strength superior to those of conventional tubes. However, a further elevation of the steam temperatures utilized and frequent run/stop operations of the boiler caused by fluctuations in the demands for electric power are anticipated, and thus a reduced plant wall thickness, i.e., a further improved creep rupture strength, is desired in order to, e.g., mitigate thermal stress. On the other hand, it is disclosed in Japanese Examined Patent Publication (Kokoku) No. 58-17820 that a W addition at 1.5% or less is effective in improving creep strength. However, it does not mention the effect of Nb. SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-strength heat-resisting ferritic steel pipe or tube having an improved creep rupture strength at a temperature of 600°C and able to be used at a higher temperature range. To achieve this object, the present inventors found that it is effective to add more than 1.5% of W, which has a high melting point and low diffusion rate, and that part of the W addition may be replaced with Mo and no change in the effectiveness for improving the creep rupture strength will result there­ from. On the basis of the above-mentioned findings, the present inventors succeeded in developing a new steel boiler pipe or tube having a superior creep rupture strength. According to the present invention, there are provided a high-strength heat-resisting ferritic steel pipe or tube which consists, in weight percentage, of: C: 0.03 - 0.15%, Mn: 0.1 - 1.5%, Cr: 8.0 - 13.0%, W: more than 1.5% up to 3.0%, V: 0.05 - 0.30%, Nb: 0.02 - 0.12% N: 0.02 - 0.05%, Mo: 1.0% or less, Si: 0.25% or less, with the remainder consisting of Fe and unavoidable impurities, and a high-strength heat-resisting ferritic steel pipe or tube which consists, in weight percentage, of: C: 0.03 - 0.15%, Mn: 0.1 - 1.5%, Cr: 8.0 - 13.0%, W: more than 1.5% up to 3.0%, V: 0.05 - 0.30%, Nb: 0.02 - 0.12% N: 0.02 - 0.05%, Mo: 1.0% or less Si: 0.25% or less, B: 0.001 - 0.008% with the remainder consisting of Fe and unavoidable ìmpurities. In a high-strength heat-resisting ferritic steel pipe or tube according to the present invention, the content of C is preferably from 0.03 to 0.12% in weight, the content of W is preferably from 1.8 to 3.0% in weight, and the content of Mo is preferably 0.5% or less in weight. A high-strength heat-resisting ferritic steel pipe or tube according to the present invention is preferably applied to steel pipe or tubes having a wall thickness of about 5 to 50 mm (about 0.2 to 2 inches). In the present invention, steel pipe is used for the traveling of high temperature fluid and has an outer diameter of about 150 to 500 mm (about 6 to 20 inches), and steel tube is used for heating, e.g., conducting heat from the outside to the inside in the boiler super heater, and has an outer diameter of about 130 mm (about 5 inches) or less. Table 1 shows four composition ranges of the steel pipes or tubes according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail. First, the reason for limiting each component covered by the present invention is described below. C is necessary for maintaining strength but is limited to 0.15% or less to maintain the weldability. That is, in accordance with the Cr content described later, these kinds of steel pipes and tubes have an extremely good hardenability such that the welding heat-affected zone hardens remarkably, which causes cold cracking upon welding. Therefore, in order to perform a complete welding, preheating at a considerably high temperature is necessary, which causes a significant decrease in the welding work efficiency. However, if the C content is kept at 0.15% or less, the maximum hardness at the welding heat-affected zone is lowered to a degree that weld cracking is easily prevented. Thus, the upper limit for the C content is set at 0.15%. When the C content is less than 0.03%, it is difficult to maintain the creep rupture strength and, therefore, the lower limit for the C content is set at 0.03%. Mn is necessary for maintaining the strength, as well as for deoxidation. The upper limit for the Mn content is set at 1.5%, as the toughness should not exceed that brought about by a content of 1.5%, and the lower limit for the Mn content is set at 0.1%, which is the minimum amount necessary for deoxidation. Cr is an indispensable element for oxidation resistance and is necessarily added to heat-resisting steels to obtain the resulting enhancement of the high temperature strength due to a fine precipitation of M₂₃C₆ and M₆C (M denotes a metal element). The lower limit for the Cr content is set at 8%, at which limit a remarkable precipitation hardening is observed, and the upper limit for the Cr content is set at 13%, from the viewpoint of weldability and toughness. W enhances the high temperature strength through solid solution strengthening and by controlling the coarsening of carbides as a solute therein, and is particularly effective for the strengthening at temper­ atures exceeding 600°C over a long term period. The lower limit for the W content is set above 1.5% since the effect sharply increases at a content above 1.5%. The upper limit is set at 3% because the weldability, toughness after aging, and oxidation resistance are impaired if an amount exceeding 3% is added. V, similar to W, remarkably enhances the high temperature strength of steel either in solid solution or in precipitation as precipitates. Particularly, when precipitation occurs, V precipitates as V₄C₃ and also partially substitutes for the M of M₂₃C₆ and M₆C. As a result, V exhibits a remarkable effect in the control of coarsening of the precipitates. However, at an amount of less than 0.05%, creep rupture strength exceeding that of AISI TYPE 347 stainless steel at around 600°C cannot be obtained, and an amount exceeding 0.30% only lowers the strength. Thus, the upper limit for the V content is set at 0.30%, and the lower limit for the V content is set at 0.05%. Nb enhances the high temperature strength through the precipitation of Nb(CN) and also contributes to the long term creep rupture strength through a primary fine-dispersion precipitation and consecutively control­ ling of the subsequent precipitation of M₂₃C₆, M₆C, etc., to form precipitates having a refined morphology. A significant effect cannot be obtained when the amount of Nb is less than 0.02%, and the strength is lowered by coalescence coarsening when the amount of Nb exceeds 0.12%. Thus, the upper and lower limits for the Nb content are set at 0.12 and 0.02%, respectively. The amount of V + Nb is preferably in the range of from 0.15% to 0.35%, from the viewpoint of creep rupture strength. N enhances the creep rupture strength through solid solution strengthening in a matrix, or by precipitating as nitrides or carbonitrides. An N content below 0.02% sharply lowers the strength, and an N content above 0.05% causes problems such as the difficulty of producing sound steel ingots, due to the generation of blow holes during casting. Thus, the upper and lower limits for the N content are set at 0.05% and 0.02%, respectively. Mo has an effect similar to that of W and effec­ tively enhances the high temperature strength, but is less effective for the refinement and coarsening-control of carbide than W. However, in the region where W content is more than 1.5%, the synergistic effect of W and Mo occurs and, therefore, the co-addition of these elements is preferable. However, an excessive amount of Mo has an adverse influence on the weldability, toughness after aging, and oxidation resistance and thus the upper limit thereof is set at 1.0%. Si is usually added for deoxidation but, in material property, has a detrimental influence on toughness. The inventors studied the influence on toughness of Si, and found that the heat embrittlement is insignifi­ cant when the amount of Si is controlled to 0.25% or less. Thus, the amount of Si is limited to 0.25% or less, preferably 0.10% or less. The steel pipe and tube according to the present invention may also contain B for further increasing the creep rupture strength. B is well known as essentially an element that remarkably enhances the hardenability, and a minute addition thereof remarkably improves the creep rupture strength. An amount below 0.001% does not have a significant effect, and an amount above 0.008% impairs the hot workability and weldability. Thus, the upper and lower limits for the B content are set at 0.008% and 0.001%, respectively. Due to the melting history, 0.3% or less of Ni and Co may be contained in steel pipe and tube as impurities, although this does not in any way impair the character­ istics of the steel pipe and tube of the present inven­ tion. In a high-strength heat-resisting ferritic steel pipe or tube according to the present invention, the content of C is preferably from 0.03 to 0.12%, the content of W is preferably from 1.8 to 3.0%, and the content of Mo is preferably from 0.1 to 0.4%, from the viewpoint of weldability and toughness. The present invention will be described in more detail with reference to the following examples, which do not limit the scope of the invention in any way. Examples Table 2 shows the chemical composition of examples of the steel tube according to the present invention, and comparative examples thereto, the creep rupture time at 650°C and 18 kg/mm², the rupture elongation, the weldability-indicated with the pre-heating temperature in constraint Y-groove cracking test (JIS Z3158), the impact valve after aging at 600°C for 1000 hours, and the tensile properties at room temperature. In Table 2, Examples 6 to 15, 17 to 19, 24, and 25 are those of the steel tubes of the present invention, Examples 1 to 5, 16, and 20 to 23 are Comparative Examples, in which Comparative Example 2 is a 2¼ Cr-1Mo steel tube, a low-alloy heat-resisting steel tube in general use, and Comparative Example 1 is an alloy steel tube used for a boiler heat exchanger, which has a further improved high-temperature corrosion resistance. The tubes of Comparative Examples 1 and 2 have a low creep rupture strength. Comparative Example 3 is a steel tube used for the superheater and reheater of a coal single-fuel combustion boiler, and has an extremely high C content compared with the Examples of the steel tubes of the present invention and, therefore, is difficult to weld and form. Comparative Examples 4 and 5 have W contents below the lower limit, and thus are lacking in creep rupture strength. Comparative Example 16 contains an amount of W above the upper limit and, therefore, has an extremely poor toughness after a long term exposure at a high temperature and an inferior weldability. Comparative Examples 20 and 21 have carbon contents outside the lower and upper limits, and thus have a lower creep rupture strength and a poor weld­ ability, respectively. Comparative Examples 22 and 23 have Mo contents above the upper limit, and the toughness thereof is very much reduced after heating. On the contrary, the steel tubes according to the present invention are considerably superior to the steel tubes of Comparative Examples 1 and 3, existing heat-­ resisting ferritic steel tubes, and can be used at considerably high temperatures under the same level of loading stress. The toughness of the steel tubes according to the present invention is on the same or at a higher level in comparison with that of an existing steel X20CrMoV121 (Comparative Example 3) and, therefore, no problems arise in practice. Additionally, Examples 10 and 11 containing 0.27% Ni and 0.26% Ni + 0.17% Co as impurities, respectively, have characteristics comparable with the other Examples of the steel tubes according to the present invention. In Examples 24 and 25, which are covered by claim 3 of the present invention, the addition of B brings a further enhancement in the creep rupture strength. In Examples 7 through 11, 24, and 25, which are covered by claims 2 and 4 of the present invention, an extremely good balance among strength, toughness, and weldability is achieved.

Claims: 1. A high-strength heat-resisting ferritic steel which is in the form of one member selected from the group consisting of pipe and tube and which consists, in weight percentage, of: C: 0.03 - 0.15%, Mn: 0.1 - 1.5%, Cr: 8.0 - 13.0%, W: more than 1.5% up to 3.0%, V: 0.05 - 0.30%, Nb: 0.02 - 0.12%, N: 0.02 - 0.05%, Mo: 1.0% or less, Si: 0.25% or less, with the remainder consisting of Fe and unavoidable impurities. 2. A high-strength, heat-resisting ferritic steel according to claim 1, wherein said C content is from 0.03 to 0.12% in weight, said W content is from 1.8 to 3.0% in weight, and said Mo content is 0.5% or less in weight. 3. A high-strength heat-resisting ferritic steel which is in the form of one member selected from the group consisting of pipe and tube and which consists, in weight percentage, of: C: 0.03 - 0.15%, Mn: 0.1 - 1.5%, Cr: 8.0 - 13.0%, W: more than 1.5% up to 3.0%, V: 0.05 - 0.30%, Nb: 0.02 - 0.12%, N: 0.02 - 0.05%, Mo: 1.0% or less, Si: 0.25% or less, B: 0.001 - 0.008% with the remainder consisting of Fe and unavoidable impurities. 4. A high-strength, heat-resisting ferritic steel according to claim 3, wherein said C content is from 0.03 to 0.12% in weight, said W content is from 1.8 to 3.0% in weight, and said Mo content is 0.5% or less in weight.

Inventors: FUJITA, TOSHIO, HASHIMOTO, KATUKUNI C/O NIPPON STEEL CORPORATION, OTOGURO, YASUO C/O NIPPON STEEL CORPORATION
Applicants: NIPPON STEEL CORPORATION
Application year: 1986
Main classification: C22C38
IPC codes: C22C38