US3809550A - Properly weldable,corrosion resistant structural steel of high yield point - Google Patents

Abstract

THIS INVENTION IS FOR CORROSION RESISTANT STRUCTURAL STEEL WHICH HAS HIGH TENSILE STRENGTH, A HIGH YIELD POINT AND WELDABLE UNDER ANY CONDITIONS. THE STEEL CONTAINS MANGANESE, NICKEL AND SILICON IN ADDITION TO THE GENERAL CONTAMINATES AND ALLOYING ELEMENTS AND NIOBIUM IN AMOUNTS THAT COPPER, ALUMINUM, NITROGEN AND NIOBIUM IN AMOUNTS THAT IMPART THE STATED PROPERITES THERETO.

Description

Int. Cl. C22c 37/00, 39/02 US. Cl. 75-124 1 Claim ABSTRACT OF THE DISCLOSURE This invention is for corrosion resistant structural steel which has high tensile strength, a high yield point and weldable under any conditions. The steel contains manganese, nickel and silicon in addition to the general contaminates and alloying elements and particularly carbon, copper, aluminum, nitrogen and niobium in amounts that impart the stated properties thereto.

The invention relates to a properly weldable corrosion resistant structural steel of high yield point, especially for constructions, machines and buildings subject to the vicissitudes of weather.

The realization of up-to-date economic investmentsfor instance light-structure buildings-necessitates the production and use of steels being weldable under any conditions by an optional welding process, with usual working plan, that is unconditionally, providing simultaneously for a corrosion resistance for several decades.

Also at present, corrosion resistant structural steels properly weldable under suitable circumstances and having good mechanical properties are known. Such steels are for instance the Cor-Ten-type steels developed and generally used in the United States.

They contain 0.10 to 0.19 percent C, 0.20 to 1.25 percent Mn, 0.25 to 0.55 percent Cu, 0.50 to 1.25 percent Cr and about 0.10 percent V, in some cases 0.07 to 0.15 percent P and about 0.50 percent Ni (under percent weight percent shall be understood). Also the Japanese Steel Fuji Corten and the home type Korell belong to this type of steel. The rate of corrosion of these steels expressed by the average penetration depth of the corrosionmakes out 0.025 to 0.054 mm. per year in industrial atomsphere. With the carbon steel tested for comparison purposes, the average penetration depth is 0.11 to 0.13 mm. under similar conditions. As for their mechanical properties, it is characteristic that their tensile strength (a amounts to 40 to 60 kp. per sq. mm., their yield point (11 to 30 to 50 kp. per sq. mm., their elongation (6 to 15 to 20 percent and their impact value to 8 to 12 mkp. per sq. cm. at +20 C. and to 2.8 to 9.0 mkp. sq. cm. at 40 C.

These steels are properly weldable, due to the risk of pentration in the heat elfect zone of the welding, however, they can be welded only under suitable conditionsfor instance preheating, subsequent heat treatment-or by special welding process. Otherwise the steel would be brittled in the heat effect zone, rendering unreliable the welded joints and limiting to a considerable extent the applicability of these steels in certain fields. A further limitation arises in case of welded constructions from the fact, that when welding chromium-containing steels to steels not containing chromium, an electrochemical corrosion occurs.

Structural steels are known the mechanical properties of which, especially with respect to the impact value stipulated as the criterion of weldability, are better than United States Patent 0 ice those of the formers. For instance the tensile strength of structural steel alloyed also with Cu, Nb and Al (0' is 66.3 to 71.2 kp. per sq. mm., its yield point (Up) 49.2 to 64.7 kp. per. sq. mm., its elongation (0' 25 to 35 percent, its impact value (A,,) 10.4 to 23.1 mkp. per sq. cm. at +20 C. and 2.0 to 11.5 mkp. per sq. cm. at 45 C., in hot-rolled condition.

There are structural steels properly weldable under suitable conditions and having high yield point, which contain in addition to the above enumerated alloying elements also V, Cr and Mo, these steels being, however, not corrosion resistant.

One group of structural steels known today has also a relatively good corrosion resistance, while due to their mechanical properties as well as to the problems mentioned in connection with their weldability their applicability is limited. The other group of structural steels while they have relatively good mechanical properties and under suitable conditions they are properly weldablehave no satisfactory corrosion resistance.

The aim of the present invention is the production of a corrosion resistant steel having a higher yield point than the known steels, and being properly weldable under any conditions, which could be used as basic material for constructions, machines and buildings as well as vehicles subject to the vicissitudes of weather.

The aim set by the invention is attained so that the steel according to the invention contains 0.155 to 0.23 weight percent of C, 0.81 to 2.50 weight percent of Cu, 0.11 to 0.20 weight percent of Al, 0.010 to 0.040 weight percent of N and 0.05 to 0.20 weight percent of Nb. Some of these alloying elements are not soluble in solid iron if being in a quantity corresponding to the composition ratio according to the invention, they do not form chemical compounds therewith, moreover, they reduce the dissolving of elements otherwise soluble in iron. Consequently, a dense protective layer hardly soluble under the efiect of moisture is formed on the surface of the steel under the effect of the atmosphere, which protects the steel against the further corrosion caused by the atmosphere and completely excludes the occurrence of pitting.

The composition of the steel according to the invention results in excellent mechanical properties and unconditional weldability and provides for the corrosion resistance. These properties develop as a consequence of the grain refining occurring under the effect of nitrides, of the precipitation hardening of Cu and Nb, as well as of the interstitial building-in of NbCN into the crystal lattice of iron and into the dislocations.

The corrosion rate of the structural steel according to the invention is-under industrial atmospheric conditions-ten times lower than that of the conventional steels and twice lower than that of the above described corrosion resistant structural steels. The corrosion product developed on the steel surface under the effect of the moisture is much less solubleas against the known corrosion resistant steelsbut the protecting layer developed in this way provides for an increased corrosion resistance. Since this protecting layerdoes not cause staining, the steel can be used as outer decorative element without painting or any other surface treatment. Moreover, the paint tolerance capacity of the steel according to the invention is also excellent.

The steel according to the invention is unconditionally weldable. The measuring number of the carbon equivalent usual for characterizing the weldability is lower than 0.55. The metallurgical structure of the steel ensures that no embrittlement and coarse structure change occur in the range of the welding heat eifect.

The steel according to the invention has outstanding mechanical property parameters, as well as advantageous weldability and corrosion properties, as a consequence of which a considerable reduction of weight can be achieved with constructions, machines and buildings.

The steel according to the invention and its mechanical, weldability and corrosion properties will be described more detailed by way of examples.

EXAMPLE 1 In a -ton arc furnace a material of the following composition has been produced:

Weight percent C 0.20 Mn I 1.33

Cr 0.12 Cu i 1.00 Nb 0.08 N 0.020 A1 0.13 Pb 0.22

This material had the mechanical, weldability and corrosion properties shown in the first column of the enclosed table when having been hot-rolled to 22 mm., aircooled, without heat treatment and those shown in the second column of the said table in case of cooling accelerated by water spray.

EXAMPLE 2 The material of the following composition has been produced similarly in a 10-ton arc furnace:

Weight percent This material had the mechanical, weldability and corrosion properties shown in the third column of the enclosed table when having been hot-rolled similarly to dia. 21 mm., then air-cooled, without heat-treatment, and

4 those shown in the fourth column in case of a cooling accelerated by water spray.

The properties of the materials introduced in the examples are shown in Table 1.

Thus, it is to be seen that the structural steel according to the invention has not only better corrosion properties than the corrosion resistant structural steels known today and not only unconditional weldability but also excellent tensile properties. It should be noted thatas against to the Cr-containing Corten-type steels, for instance-the structural steel according to the invention can be safety produced also in open-hearth furnace and in converter, being thus suitable for mass-production.

TABLE 1 Material according to- Property Unit Example 1 Example 2 Tensile strength Kp. per sq. mm..- 84.5 150 97.0 170 Yield point Kp. per sq. mm... 69.2 82.0 Elongation Percent 23 12 22 11 Reduction in area. do 67 62 Impact value at 40 C. Mkp. per sq. cm- 18 16 12 13 C equivalent/according 0.46 0.46 0.49 0.49

to ISO TC 17/SCI. Hiardrzriass or basic ma- HVm 222 240 err Hardness of the heat HV 225 273 effect zone of welding joint. Corrosion penetration Mm 0.017 0.015

depth per year in industrial atmosphere.

What we claim is:

1. Corrosion resistant structural steel of high tensile strength and yield point, properly weldable under any conditions, consisting essentially of by weight percentage, in addition to the general contaminating and alloying elements 1.0 to 2.50 percent of Mn, 0.30 to 1.90 percent of Ni, 0.02 to 0.90 percent of Si, 0.155 to 0.23 percent of C, 0.81 to 2.50 percent of Cu, 0.11 to 0.20 percent of Al, 0.010 to 0.040 percent of N 0.05 to 0.20 percent of Nb and the balance being iron.

References Cited UNITED STATES PATENTS 2,679,454 5/1954 Ofienhauer 75125 3,303,061 2/1'967 Wilson 75124 3,692,514 9/1972 Hydrean 75-124 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75-125