SE421538B - Cast steel with a high abrasion resistance - Google Patents

Abstract

The invention relates to cast steel with a high abrasion resistance for grinders and especially the grinding parts of refining tools used in the cellulose industry. The steel has the following composition: 2-4 wt-% of carbon, at most 2 wt-% of silicon, at most 2 wt-% of manganese, 20-35 wt-% of chromium, at most 5 wt-% of molybdenum, 0.5-4 wt-% of nickel, 1-5 wt-% of titanium, and at most 1 wt-% of a combination of vanadium, niobium, tantalum, zirconium and aluminium. The chromium content is suitably 26-28%, and the vanadium content is suitably 0.1-0.5%. After annealing at a temperature of over 800 degrees C and cooling to room temperature, the steel obtained is essentially free of coarse carbides. <IMAGE>

Description

8003950-5 10 15 20 25 30 35 solve this task, it is considered desirable that the wear surface of the grinding means is and remains rough in order to thereby achieve an Eordémåd power transfer from grinding segments to chips, which has an effective grinding as a result.

To satisfy these incompatible requirements, a variety of steel alloys have been developed. Some representative steel types, which together with modified variants can today be said to be dominant for milling segments for the cellulose industry, have been compiled in Table 1.

Table 1 Composition (directional analyzes), weight-Z, Steel type, commercial designation _ C Si Mn Cr Ni Mo Ti Al Ni-Hard 3 0.7 1.0 1.5 4.5 - - - uuß ° ss 122 u 1.0 0.7 0.5 17 1.0 0.7 - - 01119 011114 ”1.0 0.5 1.0 13.5 - - - - 110-110110910 1.9 1.0 0.45 10 .2 2.2 0.7 2.0 0.17 residual iron and impurities in normal levels.

The hardness in hardened and tempered condition and the roughness of cast grinding segments made of the above cast steel are shown in Table 2.

The roughness has been measured on the boom surface and is expressed as Ra number (mean surface deviation) and as mean profile depth. 'rabeu 2 Steel type Hardness Ra-value Profile depth, around Ni-Hara 50 0.21 1.5 unß®ss 122 a se 0.09 0.6 uußwnzn ”57 0.09 0.0 nc-HARDQTD ss 0.63 3 DESCRIPTION OF THE INVENTION The object of the invention is to provide further improved grinding means for the cellulose industry in particular. More specifically, one aim is to offer a non-brittle cast steel with very high abrasion resistance, good corrosion resistance and good machinability in combination with very good grinding properties.

In order to achieve these and other objects, very high-chromium-containing cast steel alloys have been tested, which have the advantage that they have good corrosion resistance due to the high chromium content. Due to a high carbon content, the steels can also be assigned high abrasion resistance. A known risk with these steels, which occur on the market, is that they may contain giant carbides, which can grow in preferential directions and make the material (super-cracked) easily fissile and sensitive to impact and fatigue. According to the invention, however, it is possible to completely prevent the coarse carbide formation by alloying the steel with an optimum content of titanium. The cast steel according to the invention is characterized in this case by the composition shown in Table 3. The stated contents refer to weight-Z.

Table 3 Primarily Preferred Most suitable area area area C 2 - 4 2.2 - 3.5 2.7 - 3.1 Si max 2 max 1.5 0.5 - l, SM max 2 max 1.5 max 1.0 Cr 20 - 35 22 - 30 26 - 28 Mo max 5 max 1 max 0,5 Ni 0,5 - 4 0,5 - 2,5 1 - 1,5 Ti 1 - 5 1 - 4 1,2 - 3 V 'Nb' Ta 'total max 1 total max 0.5 total max 0.5 Zr, Al The rest consists essentially only of iron and impurities in normal levels. Preferably, the chromium content is between 26 and 28 Z. It has also been found suitable that the steel contains 0.1 - 0.4 Z vanadium.

BRIEF DESCRIPTION OF THE DRAWINGS In the following description of some charge produced: reference will be made to the accompanying figures. In these, Fig. 1 shows the structure of a known high-chromium-containing cast steel, Fig. 2 shows the structure of a steel with the same basic composition containing about 0.7 Z titanium, and Fig. 3 shows the structure of a steel with the same basic type containing about 1, 5 2 titanium.

EXAMPLE Three test batches were made of steel having the following composition.

Table 2 Composition, weight-Zf1 C si Mn cr Ni Mo Ti v A 3.2 0.59 0.84 727.8 0.38 0.3 f - residue Fe and impurities B 2.82 1.55 0, 77 27.7 0.56 0.33 0.74 0.19 - "- C 2.66 1.35 0.80 26.8 0.81 0.32 1.49 0.22 -" - Fig. 1 shows the structure of steel A after hardening. The structural photo clearly shows that the steel contains giant chromium carbides, which have grown in certain preference directions. As a result, the material has become super brittle in the direction of growth.

Fig. 2, which shows the structure of steel B, shows that a material with about 0.7 Z of titanium still has coarse carbide growth. The material has been long-annealed (850 ° C, 5 hours) and shows the ratio in the center of a 50 kg ingot. v ~ 8003950-6 The hardness is about 815 HB.

Fig. 3, which shows the structure of a steel C according to the invention after the same treatment as steel B, shows that the coarse carbide formation has completely disappeared when the steel contains about 1.5 Z of titanium. The hardness is the same as with steel B, ie about 815 HB.

Claims (6)

aoosssß-6 10 15 20 25 30 lä PATENTKRAV Abrasion-resistant cast steel for grinding devices, in particular grinding segments for refiners in the cellulose industry, characterized in that the steel has the following composition expressed in weight ~ 2: 2-4 _c max 2 Si max 2 Mn 20 - 35 Cr Max 5 Mo 0,5 - 4 Ni «1 - 5 Ti in total max 1 zv, Nb, n, zr, A1 left essentially only iron and impurities in normal concentrations. Cast steel according to claim 1, characterized by the following composition: 2.2 - 3.5 C max 1.5 Si max 1.5 Mn 22 - 30 Cr max 1 Mo 0.5 - 2.5 Ni I - 4 Ti in total max 0.5 ZV, Nb, Ta, Zr, A1 left essentially only iron and impurities in normal concentrations. Cast steel according to claim 2, characterized by the following composition: 2.7 - 3.1 C 0.5 - 1.5 Si max 1.0 mn 25 - 30 Cr max 0.5 Mo 1 - 1, 5 Ni 10 15 8003950-6 1, 2 - 3 Ti total max 0.5 ZV, Nb, Ta Zr, A1 essentially only iron and impurities in normal levels. Cast steel according to one of Claims 1 to 3, characterized in that the chromium content amounts to 26 to 28 Z. Cast steel according to one of Claims 1 to 4, characterized in that the steel contains 0.1 to 0.5 Z of vanadium. c Cast steel according to one of Claims 1 to 5, characterized in that the steel, after annealing at a temperature above 800 ° C and cooling to room temperature, is substantially free of coarse carbides.