GB2164057A - High carbon, low alloy steel comprising niobium - Google Patents

Abstract

The steel has improved toughness and wear resistance as a result of the addition of 0.01 to 1.00% by weight of niobium. A first embodiment comprises 0.70 to 0.90% by weight of C; 0.80 to 1.10% by weight of Mn; 0.15 to 0.30% by weight of Si; 0.025% (maximum) by weight of P; 0.015% (maximum) by weight of S; 0.40 to 0.60% by weight of Cr, and, in this invention, including from 0.01 to 1.00% by weight of Nb and a second embodiment, comprises 0.70 to 0.90% by weight of C; 1.30 to 2.00% by weight of Mn; 0.15 to 0.30% by weight of Si; 0.025% (maximum) by weight of P; 0.015% (maximum) by weight of S, and, in this invention, 0.01 to 1.00% by weight of Nb.

Description

SPECIFICATION Improvement in steels for the manufacture of disks and other agricultural tools which work on the soil This invention relates to the development of steels of the type with high carbon content and low alloy used for the manufacture of disks and other active elements for agricultural machines, characterized by having niobium in their composition, thereby turning said disks and other active members highly wear-resistant and giving them extraordinary good conditions to absorb adequately the impacts which occur during their work in the field, i.e, assuring a high toughness.

Further, as it is well known, the disks used with agricultural equipment are the active members which perform the soil revolving and/or the field's clearing from weeds. Accordingly, they are steadily subject to work under the most diverging friction or load charge conditions, which actually do not allow a longer time of use. This is the reason for which the manufacturers of these equipments are persistently engaged in improving their efficiency and in lengthen their useful life, mainly if the relatively high replacement cost is considered, really very expensive and which, in some implements, has to be made within a few crops.

Prior art agricultural disks are usually obtained from steel plates of different compositions, such as cited, for instance, in the technical norm No. NBR 6192/1983 of ABNT (Brazilian Association of Technical Standards): HOT ROLL SIEXL PLATE FOR MM;EACTUS OF AGRICULTtISIL qUIPT Element (%by weight) Designa- tion Carbon Manganese Silicon Chromium Vanadium 59DA 0.55-0.65 0.80-1.10 0.15-0.35 0.90-1.20 0.07-0.12 70DA3 0.65-0.75 0.60-0.90 0.15-0.30 --------- 70DA6 0.65-0.75 0.60-0.90 0.15-0.30 0.15-0.30 70DA9 0.65-0.75 0.60-0.90 0.15-0.30 0.40-0.60 80DA 0.75-0.88 0.60-0.90 0.15-0.30 --------- 88DA3 0.83-0.93 1.10-1.40 0.15-0.30 88DA6 0.88-0.93 1.10-1.40 0.15-0.30 --------- 88DA9 0.83-0.93 1.25-1.55 0. .15-0 .30 0.15-0.30 comprising the maximum content of Phosphor=0.040 and Sulfur=0.40% for plates having been cross rolled; or maximum content of Phosphor=O.025% and Sulfur=0.015% for plates treated by the unidirectional roll process.

The composition designated as 70 DA9, though with the same drawbacks as of the others, is the one that is of greater use in the manufacture of steel plates directed to the production of disks and other tools for agricultural machines.

With respect to their manufacturing from the known plates, the disks are molded in different forms or shapes, all of them passing through the steps of cutting, perforation, heating up to a temperature of 950 to 980 C, stamping, and then being hardened in oil, annealed in a temperature range from 480" to 580 C, sharpened and painted, in general by immersion.

As per the qualities attributed to the disks for the work they perform, the composition of the plate and the heat treatment steps are of relevant significance, since the hardening and annealing of the disks determine the final structure and the mechanicai properties of same.

Turning now to the prior art compositions, it results that the utilization of one or other presents serious inconvenients, among which the following are to be detached: -variation of the toughness as a result of the relatively high heating (950 -980 C), what results in the increase of the austenitic grain, in a continuous or discontinuous manner (duplex grains), which diminishes greatly the impact resistance; -a low carbide (Fe3C) percentage is presented by the material after the hardening and annealing operations, what has the trend to a pronounced wearing by abrasion with the soil.

In face of these circumstances and with the purpose to overcome the same, several features relating to the composition and the thermal treatment phases were analysed, such as hardening and annealing, and as a result of which the improvement object of this invention was developed and which consists essentially in the inclusion of niobium (Nb) in the composition of steels of the type with a high carbon content and a low alloy for use in the manufacture of disks and other agricultural tools working in the soil, which, by their acting form, brought new properties to disks and other active members os agricultural machines, as hereinbelow mentioned.

With the purpose of explaining the improvement in steels for the manufacture of disks and other tools for implements which operate in the soil, object of this patent application, reference is made to the accompanying drawings, in which: Figure 1 shows the austenitic grain size of the steel type ABNT 70DA9, normally employed (micrographic test-magnified by 100); Figure 2 shows the austenitic grain size of steel with niobium (micrographic test-magnified by 100); Figure 3 is a comparative graphic between the increase of the austenitic grain of the ABNT 70DA9 steel and the steel including Nb; Figure 4 is a comparative graphic of the weight loss between the conventional composition 70DA9 and compositions A, B, C, and D, object of the invention, when submitted to the wearing test;; Figure 5 shows the results of the "Sphere Test" with the steel ABNT 70DA9 with thermo treatment-Hardness Rockwell c 40-41; Figure 6 shows the results of the "Sphere Test" with steel including niobium and thermo treated-Hardness Rockwell c 40-41; Figures 7 & 8 are graphics representing the "Indurateability Test By Hardening, For Steel, According to ASTM A 245-45T", to which the steels of the invention were submitted; and Figure 9 is a graphic comparing the hardness of a annealed piece hardened at 960 C in oil, of the ABNT 70DA9 usual steel, and of steels with niobium pursuant this invention.

In accordance with the subject matter of this invention, the improvement in steels for the manufacture of disks and other agricultural tools which work in the soil consists essentially in the inclusion, in the composition of a steel having a high carbon content and a low alloy, of the type as employed to produce disks and other active members of agricultural machines which work on the soil, a correspondent portion of niobium (Nb) in a low or high concentration.

In a comprehensive manner, the presence of Nb in steels for this purpose may be defined by one of the following forms: A B C D C 0.70-0.90 0.70-0.90 0.70-0.90 0.70-0.90 Mn 0.80-1.10 0.80-1.10 1.30-2,00 1.30-2.00 Si 0.15-0.30 0.15-0.30 0.15-0.30 0.15-0.30 P 0.025(max.) 0.025(max.) 0.025 0.025(Fnax.) 0.025(max.) S 0.015(max.) 0.015(max.) 0.015(max.) 0.015(max.) Cr 0. 40-0 .60 0-40-0 .60 0.40-0.60 0-40-0.60 Nb 0.01-0.10 0.10-1.00 0.01-0.10 0.10-1.00 The niobium, as the key element of the new composition, assures the elimination of the great or duplex grain (which causes variation of toughness), since its action as microbonding element acts as an efficient grain refiner, maintaining a fine granulation ASTM 8 and lower than 8 up to the austenitation temperature of 1,100 C, approximately. This enlarges with great advantages the normally restricted temperature range, compulsorily used for the austenitation of common steels employed heretofore. Figs. 1 and 2 show clearly the effects of the austenitation temperature on the steel of the ABNT composition 70DA9 (Fig. 1) and on the steel including niobium (Fig. 2).It is noted that the latter, with niobium, presented a much finer granulation at the austenitation temperature of 900"C.

In Fig. 3 appears the effect of austenitic grain growing for the ABNT 70DA9 steel and for steel with niobium. This grain refining results in a noticeable toughness increase against impact, an increase of the resistance limit, improvement in fatigue resistance of plow and grader disks.

Also, as this is well known, a fine grain results in a noticeable reduction of microcrackings, both transgranular and intergranular, in the martensite plate type (high carbon), which for a same carbon content is highly dependent on the austenitic grain size, as taught in "Microcracking Sensitivity in Fe-C Plate Martensite' '-A.R. MARDER, A.O. BENSCOTER, AND G. KRAUSS-Me- tallurgical Transactions, Volume 1, June 1970, 1545; and, further, in "The Effect of Austenite Grain,Size or Microcracking in Martensite of an Fe-1.22C Alloy"--R. P. BROBST AND G.

KRAUSS-Metallurgical Transactions, Volume 5, February 1974-457.

It must be added that in addition to the fact that the niobium operates as grain refiner, there further occurs, after the hardening and annealing, a precipitation of niobium carbons (NbC) in a discontinuous and finely dispersed mode, or of carbonitrides of nobium (NbN), thereby improving the wearing resistance by abrasion, being also known that fine grains have a greater resistance against abrasion than coarse grains besides elevating the toughness.

Fig. 4 shows the results obtained in the wearing tests made with the compositions developed with Nb A B C D, as compared with the ABNT 70DA9 alloy, each result representing an average of six runs. In the corresponding figure, it may be noted that all the alloys developed, both with low and high niobium content, presented a smaller mass loss than the ABNT 70DA9 steel; thus being characterized by a wearing resistance by abrasion for the alloys now developed.

The wearing tests by abrasion were run by a dry gliding operation against an abrasive disc attached to an aluminium disc having a speed of 300 rpm, a charge or load of 800 g, and a duration period of 25 min. The test samples of the steels under study were thermo-treated to a hardness in the range of 40 to 41 Rockwell-c. After the heat treatment the test samples have been machined to the standard size of 10 mm in diameter by 20 mm of lenght before being tested.

The improvement of toughness was verified by means of the "Sphere Test" according to the Standard Project ABNT No. 12.02.05.004-DISK FOR AGRICULTURAL MACHINES-SPHERE TEST-Testing Method, which results are presented in Figs. 5 and 6 for the ABNT 70DA9 steel, and for the steel with niobium, respectively. It was noted that the 70DA9-steel presented after the test (Fig. 5) a breaking away of material, and the niobium-steel (Fig. 6) presented a simple plastic deformation, necessary to the sphere's passage, thereby evidencing a greater toughness.

The effect of niobium on the hardening ability was evaluated through the "Jominy" test, there having been noted that it has practically no effect on the hardening ability increase, both in the compositions presented and comprising Chromium and Manganese, and as well in compositions which contain Manganese only.

Figs. 7 and 8 are graphics representing the "Indurateability Test by Hardening for Steel", in accordance with the ASTM Standard 245-45T, where in Fig. 7 the composition used is of 0.75% C, 0.91% Mn, 0.26% Si, 0.021% P, 0.02% S, 0.63% Cr, and 0.02% Nb; while in Fig. 8 the composition used is of 0.73% C, 1.50% Mn, 0.27% Si, 0.020% P, 0.017% S, and 0.41% Nb.

As seen, among the alloy compositions for the manufacture of disks and agricultural tools, usually employed, the most outstanding ones are those which have in their composition Mn and Cr in low quantities as well as others which have only Mn in high quantities, but the first ones, due to the use of the two elements, have a greater annealing brittleness than the alloys containing only one element.

The use of niobium as now proposed could diminish or even prevent, as a secondary effect, the brittleness of the annealing, and as a result it could well replace vanadium and molybdenum, normally employed to prevent the brittleness of the annealing.

Thus, all the compositions tested presented, practically, the same hardening ability and toughness after the annealing of samples hardened at 960"C in oil, as this is shown in the graphic of Fig. 9.

Claims (4)

1. An improvement in steels for the manufacture of disks and other agricultural tools which work on the soil of the high carbon content and low alloy type characterised by the inclusion of 0.01 to 1.00% by weight of Niobium in the steel composition.

2. An improvement in steels as claimed in claim 1 wherein the steel further includes 0.70 to 0.90% by weight of Carbon, 0.80 to 1.10% by weight of Manganese; 0.15 to 0.30% by weight of Silicon; 0.025% (maximum) by weight of Phosphorus; 0.015% (maximum) weight of Sulphur and 0.40 to 0.60% by weight of Chromium.

3. An improvement in steels as claimed in claim 1 wherein the steel further includes 0.70 to 0.90% by weight of Carbon; 1.30 to 2.00 by weight of Manganese; 0.15 to 0.30% by weight of Silicon; 0.025% (maximum) by weight of Phosphorus and 0.015% (maximum) by weight of Sulphur.

4. An improvement in steels for the manufacture of disks and other agricultural tools which work on the soil substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.