AU2013302197A1

AU2013302197A1 - Method for producing molten steel having high wear resistance and steel having said characteristics

Info

Publication number: AU2013302197A1
Application number: AU2013302197A
Authority: AU
Inventors: Ricardo LEIVA ILLANES; Raoul MEUNIER ARTIGAS
Original assignee: Electro Metalurgica SA
Current assignee: Electro Metalurgica SA
Priority date: 2012-08-09
Filing date: 2013-07-25
Publication date: 2015-03-26
Anticipated expiration: 2033-07-25
Also published as: PE20150740A1; CN104822847B; WO2014022944A1; CA2886286A1; AU2013302197B2; CL2012002218A1; US20150252457A1; CA2886286C; CN104822847A

Abstract

The invention refers to a method for producing a molten steel having high wear resistance having a mainly bainitic microstructure and a suitable balance of tensile strength and hardness for pieces of large size in mining operations such as milling and grinding, the chemical composition of which, expressed in weight percentage, comprises: 0.30-0.40% of C, 0.50-1.30% of Si, 0.60-1.40% of Mn, 2.30-3.20% of Cr, 0.0-1.00% of Ni, 0.25-0.70% of Mo, 0.0-0.50% of Cu, 0.0-0.10% of A, 0.0-0.10% of Ti, 0.0-0.10% of Zr, less than 0.050% of P, less than 0.050% of S, less than 0.030% of N, optionally less than 0.050% of Nb, optionally 0.0005-0.005% of B, optionally 0.015-0.080% of rare earth metals, and residual contents of W, V, Sn, Sb, Pb and Zn of less than 0.020%, and the balance in iron. The method for producing the molten steel includes smelting and heat treatment. The smelting can be carried out in an electric arc furnace having basic or acid refractory or an electric induction furnace. Smelting in an electric arc furnace as a normal operation includes melting, oxygen insufflation, blocking, refining and deoxidation. Smelting in an electric induction furnace includes melting, refining, control of nitrogen in solution and deoxidation. Heat treatment comprises settling and tempering. The molten steel described in the invention exhibits a suitable balance of the chemical composition, tensile strength and hardenability to assure complete hardening in cast pieces of large size, typically up to 17 inches in thickness, with Brinell hardness preferably in the range of 385-495 BHN throughout the section of the piece and excellent resistance to wear by abrasion impact.

Description

WO 2014/022944 PCT/CL2013/000047 METHOD FOR PRODUCING CAST STEEL HAVING HIGH WEAR RESISTANCE AND STEEL HAVING SAID CHARACTERISTICS Field of application 5 The present invention relates to the field of wear resistant metallic materials, especially cast steels resistant to wear by abrasion and impact for mining applications. More particularly, the present invention 10 relates to a method for producing cast steel, by which a wear-resistant steel is obtained, with predominantly bainite microstructure and a suitable balance of toughness and hardness for use thereof in mining applications, such as grinding, crushing and all those 15 applications that require large components with high resistance to wear by abrasion and impact. Even more particularly, the present invention relates to a cast steel of predominantly bainite structure, with a suitable balance of toughness and hardness and 20 resistant to wear, to be used in the applications mentioned above. The technical problem 25 Various methods for producing steels for mining applications are known in the prior art. However, the useful life of the components obtained by these methods is unable to meet production requirements. In particular, the known methods do not provide steels 30 whose hardenability is sufficient to ensure high hardness over the entire cross section of components of large thickness made with this steel. Solutions in the prior art 35 No methods have been identified for producing cast steels that are able to provide an alloy with the necessary hardenability and hardness for use thereof in mining applications that require large components with WO 2014/022944 - 2 - PCT/CL2013/000047 high resistance to wear by abrasion and impact, such as grinding and crushing; and with increased resistance to wear by abrasion and impact, such as is provided by the present invention. 5 In general terms, the cast steels that are usually employed in the aforementioned mining applications may be classified as: i) austenitic steels of the Hadfield type; ii) low-alloy Cr-Mo steels with predominantly 10 pearlitic structure; and iii) low-alloy steels with low to medium carbon content with predominantly martensitic microstructure. None of these steels effectively solves the aforementioned problems, as is explained in detail hereunder. 15 The austenitic manganese steels of the Hadfield type, such as those described in standard ASTM A128, are produced by heat treatment for solution of carbides and water quenching, obtaining a Brinell hardness in the 20 as-heat-treated condition of about 200 BHN. Moreover, these cast steels possess a high capacity for hardening by cold working, and may reach a hardness on the worked surface of up to 450 BHN. Moreover, in view of the increased toughness of these steels, they are mainly 25 used in coatings for ore crushing equipment. However, when the mechanical stress is not sufficient to produce high hardening by cold working, austenitic manganese steels inevitably display low abrasive wear 30 resistance, greatly reducing the useful life of components made with said steels. For their part, low-alloy Cr-Mo steels with predominantly pearlitic microstructure are made by a 35 normalizing and annealing heat treatment, reaching Brinell hardnesses in the range 275-400 BHN. These steels have been widely used as cladding for SAG mills over the course of the last 30 years with acceptable WO 2014/022944 - 3 - PCT/CL2013/000047 results, without undergoing large modifications. Despite the foregoing, owing to the global trend in the mining industry to use ore processing equipment of 5 larger size, added to the ever-increasing mechanical stress to which the components are subjected, the "acceptable results" currently obtained with Cr-Mo steels are inadequate. In view of this, the use of low alloy Cr-Mo steels with predominantly pearlitic 10 microstructure is limited, since it is not possible to increase their wear resistance by increasing the hardness, without having an adverse effect on toughness. Consequently, the use of these alloys under the current conditions inevitably increases the 15 probability of failure. Finally, another type of steel commonly used in the mining industry corresponds to the low-alloy steels with low to medium carbon content with predominantly 20 martensitic microstructure. These steels are produced by a heat treatment of hardening and annealing, reaching Brinell hardnesses in the range 321-551 BHN, depending on the specific carbon content of the alloy and the conditions employed in heat treatment. At 25 present, these steels are widely used in cavities of crushers, shovel teeth of earth-moving machinery, discharge chutes and antiwear plates, all these components having thicknesses typically of less than 8 inches (20.3 cm) . However, since these steels do not 30 possess sufficient hardenability, it is not possible to guarantee constant high hardness through the cross section of the component, from the surface to the center, for components with thicknesses above 6 inches (15.2 cm) . To solve the above problem, increasing the 35 content of carbon and of alloying elements has been tried. However, it has been found that this route causes a considerable decrease in toughness. Moreover, low-alloy steels with low to medium carbon content WO 2014/022944 - 4 - PCT/CL2013/000047 require a greater cooling rate to obtain a martensitic structure, usually employing water, oil or forced air as the quenching medium. This not only gives rise to higher costs of manufacture, but also hampers the 5 production of large components or those with complex geometry with large changes of section. Thus, although in the prior art there are methods for producing steels for mining applications, the inventors 10 have not detected any disclosure of a method capable of producing a cast steel of the composition and microstructure specified in the present invention and which in addition offers the aforementioned advantages. 15 As an example, document JP 2000 328180 of Tamura Akira et al. relates to a wear-resistant cast steel of predominantly martensitic microstructure, to be used in components of mills used by the cement industry, ceramic industry, etc. Both the chemical composition 20 and the microstructure of this steel are substantially different from those of the steel obtained by the method of the present invention. The steel described in JP 2000 328180 has a chromium content preferably in the range 3.8-4.3% w/w. Moreover, said document teaches 25 that a chromium content below 3.0% w/w adversely affects the hardenability of the steel. In contrast, the present invention describes steels with predominantly bainite microstructure with chromium concentrations in the range 2.3-3.2% w/w and with 30 adequate hardenability and hardness in large components. In addition, the steel described in document JP 2000 328180 does not disclose microadditions of 35 titanium and zirconium, as envisaged in the present invention. This document also does not disclose optional additions of niobium, boron and/or rare earths.

WO 2014/022944 - 5 - PCT/CL2013/000047 Moreover, document JP 09 170017 of IIHARA Katsuyuki et al. relates to a rolled steel of high strength and toughness that has a predominantly bainite 5 microstructure. However, both the chemical composition of this steel and the method for producing it differ from those disclosed for the steel obtained by the method of the present invention. As an example, the steel described in JP 09 170017 has a higher carbon 10 content and a lower content of silicon and manganese than the steel of the invention. Moreover, it has addition of vanadium for controlling grain size. Although the bainitic steel of high strength and 15 toughness described in JP 09 170017 uses microalloying elements to obtain a fine bainite microstructure, it has a lower content of silicon and manganese to ensure high toughness, and accordingly it does not develop sufficient hardness, hardenability and wear resistance 20 for use in conditions of abrasion and severe impact in mining operations. US patent 7,662,247 of HU Kaihua discloses wear resistant cast steels with a predominantly martensitic 25 microstructure that includes films of austenite for improving toughness, and the method for producing same. For its part, US patent 3,973,951 of SATSUMABAYASHI Kazuyoshi et al. discloses a cast steel of high wear resistance and toughness for use as nails, tips, blades 30 or other tools for excavation in construction industry machinery. Although both documents disclose steels with increased toughness, the high silicon concentration in these 35 steels (1.40-2.05% w/w) has an adverse effect on the manufacture of components with large thickness, since it promotes the occurrence of phenomena of hot cracking during solidification of the components.

WO 2014/022944 - 6 - PCT/CL2013/000047 Additionally, patents US 5, 382, 307 of KAGEYAMA Hideaki et al., US 5,676,772 of KOBAYASHI Kazutaka et al. and US 6,254,696 of UEDA Masaharu et al. describe steels 5 used for making railway tracks with high strength and toughness, resistant to contact fatigue, and that are manufactured by a process of melting, hot rolling and normalizing in forced air. These steels differ from the steels of the present invention in that, although they 10 possess high toughness, they do not have a suitable balance of chemical composition that allows them to obtain a high hardness that is practically constant through the cross section in components with large thickness, despite the fact that high contents of 15 manganese, silicon and/or nickel are specified. Finally, the steel obtained by the method of the invention also differs from other bainitic steels, such as the carbide-free steels described in US 2010/0294401 20 of Gonzalo Gomez et al. and US 5,879,474 of BHADESHIA Harshad et al. In contrast to the steel of the invention, the carbide-free bainitic steels of these documents have contents of manganese, silicon and/or aluminum above 1.50% w/w for promoting the presence of 25 bainite and inhibiting the precipitation of cementite, and moreover have a microstructure with high contents of retained austenite. This retained austenite could optionally be transformed to martensite under the action of events with severe impact, causing phenomena 30 of surface fatigue with large losses of material by a mechanism of accelerated wear known as spalling. The present invention provides a bainitic cast steel that overcomes all the drawbacks mentioned above, since 35 it possesses suitable wear resistance and a suitable balance between toughness and hardness, and is useful in mining applications that require large components with high resistance to wear by abrasion and impact, WO 2014/022944 - 7 - PCT/CL2013/000047 especially those associated with crushing and grinding. Brief description of the invention 5 The method and the steel of the present invention provide a solution to the limitations described above displayed by conventional wear-resistant steels that are used at present and do not provide a suitable balance between high hardness, hardenability, toughness 10 and wear resistance in components with large thickness, typically up to 17 inches (43.18 cm). The present invention overcomes these drawbacks with a method for producing steel that provides a cast steel 15 of predominantly bainite structure with high hardness that is practically constant through the cross section in components with large thickness, which translates into high resistance to wear by abrasion and impact, maintaining a suitable balance between its hardness and 20 toughness. One of the aims of the present invention is to provide a cast steel whose hardenability is sufficient to ensure high hardness over the entire cross section of 25 components with large thickness or components of complex geometry with large changes of section, used in mining applications that require large components with high resistance to wear by abrasion and impact, such as grinding and crushing, thus increasing the useful life 30 of the components. Brief description of the figures For the purpose of describing the method of the present 35 invention with greater clarity, a detailed description of the invention is provided below, with examples of application, which are illustrated in the accompanying figures, where: WO 2014/022944 - 8 - PCT/CL2013/000047 Fig. 1 is a block diagram of an embodiment of the present invention, where the solid lines represent the main steps of the present invention. 5 Fig. 2 illustrates the typical bainite microstructure of the steel obtained by the method of the present invention. Reagent Nital 5%, at 40OX. 10 Fig. 3 corresponds to a continuous cooling diagram (CCT, abbreviation for continuous cooling transformation) determined for one of the steels described in the present invention. 15 Fig. 4 is a curve describing the kinetics of precipitation of particles of second phase of a GS-35 CrMoV 10 4 steel. 20 - Fig. 5 is a curve describing the kinetics of precipitation of particles of second phase of one of the steels described by the invention. * Fig. 6 is the profile of Brinell hardness 25 evaluated from the surface to the center of components made from a conventional pearlitic CrMo steel and a steel described by the invention. 0 Fig. 7 is a graph showing the thermal cycle of 30 normalizing and annealing according to a typical application of the present invention. Detailed description of the invention 35 One of the aims of the present invention is to provide a method for producing bainitic cast steel having high wear resistance and exhibiting greater hardenability than the steels known in the prior art.

WO 2014/022944 - 9 - PCT/CL2013/000047 Another aim of the present invention is to provide a method for producing steel with high hardness over the entire cross section of components made therewith, 5 especially those of large size. Another aim of the present invention is to provide a bainitic cast steel having high wear resistance, with a suitable balance of toughness and hardness. 10 Yet another aim of the present invention is to provide a method for producing a cast steel with a suitable balance between chemical composition, toughness and hardenability; and a steel with said characteristics. 15 Another aim of the present invention is to provide large steel components for mining applications, such as crushing, grinding and all those applications that require large components with high resistance to wear 20 by abrasion and impact, whose useful life is greater than that of components of the prior art; and a method for producing said steel. The bainitic steel with increased toughness of the 25 present invention has the following chemical composition: * 0.30-0.40% w/w C e 0.50-1.30% w/w Si 30 e 0.60-1.40% w/w Mn * 2.30-3.20% w/w Cr, more preferably 2.40-3.0% w/w Cr * 0.0-1.00% w/w Ni * 0.25-0.70% w/w Mo 35 e 0.0-0.50% w/w Cu e 0.0-0.10% w/w Al e 0.0-0.10% w/w Ti e 0.0-0.10% w/w Zr WO 2014/022944 - 10 - PCT/CL2013/000047 * Less than 0.050% w/w P * Less than 0.050% w/w S * Less than 0.030% w/w N e Optionally less than 0.050% w/w Nb 5 e Optionally 0.0005-0.005% w/w B * Optionally 0.015-0.080% w/w rare earths * Residual contents of W, V, Sn, Sb, Pb and Zn less than 0.020% w/w and the balance iron. 10 Unless stated otherwise, the concept "Rare earths" preferably refers herein to commercial mixtures of cerium and lanthanum. Some of the basic criteria considered for limiting the 15 chemical composition in the range described by the present invention were as follows: e The carbon content is essential for determining the hardness of steel. Carbon contents under 0.30% 20 w/w are insufficient to obtain hardening by solid solution, high hardenability and hardening by precipitation of complex carbides or carbonitrides that guarantee practically constant hardness in large components and high wear resistance; whereas 25 carbon contents above 0.40% w/w have an adverse effect on impact toughness in bainitic-martensitic steels. * Silicon increases the strength of steel by solid 30 solution hardening of the ferritic matrix of the bainite structures and delays the precipitation of carbides, so that it prevents abrupt decrease of hardness during annealing. However, silicon contents above 1.30% w/w have an adverse effect on 35 the manufacture of components with large thickness, promoting the occurrence of phenomena of hot cracking.

WO 2014/022944 - 11 - PCT/CL2013/000047 * Manganese causes a moderate increase in hardenability of steel and refines bainite structures. However, at contents above 1.40% w/w it displays pronounced interdendritic chemical 5 segregation, especially in large components. * Chromium is an important element that provides strength, hardenability and hardening by precipitation of alloyed carbides of the M7C3 and 10 M23C6 type. The inventors concluded that the range previously defined for chromium will produce a suitable balance of hardness, hardenability and distribution of chromium-rich alloyed carbides that ensure high wear resistance. 15 * Molybdenum is an important element that provides strength, high hardenability and hardening by precipitation of carbides of the M6C type and carbonitrides of the M(C,N) and M2(C,N) type. 20 Moreover, it greatly reduces the harmful effect of impurities that may segregate at grain boundaries, causing embrittlement. For this reason a minimum molybdenum content of 0.25% w/w is stipulated. However, in view of its high cost, it is desirable 25 to limit its content to a maximum of 0.70% w/w. * Nickel increases the cohesion energy of the grain boundary, promotes the presence of bainite structures to the detriment of pearlite and has a 30 synergistic effect on additions of manganese and molybdenum. However, it also has a high cost and its addition must be limited. * Apart from having a deoxidizing effect, additions 35 of titanium and zirconium allow nitrogen to be fixed in solid solution, control the grain size and provide hardening by precipitation of carbonitrides of the M(C,N) type. For its part, WO 2014/022944 - 12 - PCT/CL2013/000047 zirconium modifies the morphology of sulfide inclusions. * Additions of rare earths, specifically mixtures of 5 cerium and lanthanum, have an important effect on refinement of casting microstructure and on modification of the morphology of sulfide inclusions in steel. Moreover, they increase resistance to surface fatigue. 10 * Additions of boron greatly increase hardenability and refine the acicular phases (bainite and martensite). However, they may have an embrittlement effect when combined with nitrogen 15 and form insoluble precipitates of BN at grain boundaries. Accordingly, the amount to be added and the sequence must be controlled in the ranges defined above. 20 e It has been found that the appropriate use of multicomponent master alloys that contain boron, titanium, zirconium, rare earths and particular mixtures thereof, together with controlled addition of these elements, ostensibly improves 25 the properties of cast steels having high wear resistance for the mining applications described in this invention. The method of production of the present invention 30 provides a bainitic steel with the chemical composition detailed above that comprises the following steps: 1. Melting: can be carried out by any conventional method. For example, this operation can be 35 performed in an arc furnace with basic or acidic refractory, or in an induction furnace. Melting in an arc furnace, as a normal operation WO 2014/022944 - 13 - PCT/CL2013/000047 comprises complete melting of the charge, followed by blowing oxygen in, to produce oxidation of the liquid metal, transfer of impurities to the slag and decarburization of the metal to remove the 5 nitrogen and hydrogen in solution. Then the operation of blocking of the liquid metal is carried out to stop oxidation, followed by the operation of refining and adjustment of the chemical composition to the specified range. Next, 10 an operation of deoxidation is carried out using aluminum and master alloys of titanium and/or zirconium. Deoxidizing elements will be added in suitable amounts so that the residual contents of aluminum, titanium or zirconium are within the 15 specified range for the alloy. If addition of boron and/or treatment with rare earths is required, this is performed in the ladle. For its part, melting in an induction furnace as a 20 normal operation comprises complete melting of the metal charge up to a temperature not above 1700'C, followed by adjustment of the chemical composition; followed by addition of master alloy of an element that is a strong nitride former 25 preferably titanium - to form a slag with a high capacity for nitrogen. Then the slag formed is removed and next the operation of deoxidation and discharge of the metal into a ladle is carried out. 30 2. Heat treatment: the normal operation of heat treatment applied to noncritical components comprises normalizing and annealing. 35 Normalizing is performed at a temperature in the range from 950 to 1050'C, for a holding time of between 3 and 10 hours depending on the characteristic thickness and geometry of the WO 2014/022944 - 14 - PCT/CL2013/000047 components to be produced. Then the components are submitted to a cooling step from the normalizing temperature to a temperature in the range from 500 to 80 0 C, more preferably between 500 and 150 0 C. 5 Cooling may be carried out either in still air or direct or indirect forced air, or a combination of both types of cooling, whenever the cooling rates of the center and surface of the component are within the range 0.050-0.50 0 C/s, so as to ensure 10 optimal phase distribution. Immediately following normalizing, an annealing heat treatment is carried out at a temperature in the range 450-630 0 C, for a time of between 3 and 15 10 hours depending on the geometry of the component and the range of hardness that is to be reached. The annealing heat treatment has the aim of achieving maximum possible transformation of the austenite, annealing the acicular phases 20 formed and producing secondary hardening by precipitation of alloyed carbides predominantly rich in molybdenum. As has been mentioned, the cast steel of predominantly 25 bainite structure (like that shown in Fig. 2) that is obtained by the method of the present invention, and that comprises the chemical composition detailed above, has a number of advantages over other steels of the prior art. One of these advantages is the high hardness 30 of the steel obtained, which is attained, among other factors, owing to the absence of phenomena of enlargement and coalescence of precipitates during a normal annealing cycle, as shown in Fig. 5. 35 In contrast, it can be seen from Fig. 4 that the steels of the prior art, such as CrMoV, usually exhibit an abrupt decrease in hardness, which could promote the occurrence of phenomena of embrittlement during WO 2014/022944 - 15 - PCT/CL2013/000047 annealing. In particular, this figure illustrates the kinetics of precipitation of particles of second phase of a GS-35 CrMoV 10 4 steel, according to standard DIN 17205, which specifies hardened and annealed cast 5 steels for general applications. Although this steel has a chemical composition somewhat similar to that of the present invention, it displays rapid enlargement and coalescence of cementite and carbonitrides of the M2(C,N) type, affecting its hardness. 10 Another advantage of the present invention is that the increased hardness is constant through the cross section of a component of large thickness, which is not achieved with steels of the prior art, as can be seen 15 in Fig. 6. In accordance with the foregoing, the cast steel obtained by the method of the present invention exhibits a suitable balance of chemical composition, 20 toughness and hardenability to ensure complete hardening in castings of large size, typically up to 17 inches (43.18 cm) in thickness, with Brinell hardness preferably in the range 385-495 BHN throughout the cross section of the component and excellent resistance 25 to wear by abrasion and impact. Embodiment examples Various tests of the method of the present invention 30 were carried out, using chemical compositions within the ranges that are disclosed here. In the following, a conventional Cr-Mo pearlitic steel, widely used in coatings for SAG mills, is compared 35 against five examples of steels obtained by the method of the present invention. The tests were performed in the operating conditions WO 2014/022944 - 16 - PCT/CL2013/000047 presented in Tables 1 and 2. Table 3 shows the chemical compositions used in each case, expressed in % w/w. Finally, Table 4 shows the phase distribution and hardnesses obtained in the heat treatment conditions 5 applied, whose cooling rate corresponds to that typically encountered in components of large thickness. Fig. 7 shows a diagram of the thermal cycle used in this example, where segment (a) describes the step of heating the components to the normalizing temperature. 10 Segment (b) shows a holding time at the normalizing temperature for 4 hours. For its part, segment (c) represents the step of cooling in air from normalizing to a temperature of 200 0 C, at an average cooling rate as indicated in Table 2. Segment (e) shows a holding 15 time at the annealing temperature of 5 hours.

oO 0OUOD)- U U oD 0HC 0 w4- 0 u 0 4CD u u 0) 4 ) 0 0 CDa 4) ) (0 U) r > CD 0r D FC o ~~ ) 0 C L c 0 000)U)H 02L a)) $r 0 u 0 0 w C 0 -4~ 4- ~ c ) 0 0 C C) F) [F O 4 - [-) (0 ND 0O a) X C4 CO4-D N H -H ___U_)_ -H C -H U 04 0 'I4 r i > u 0 0 w a)C 0 0 C H OO > 4-) rCC 4- o> NH 1 4 -- 4 a)) C] 0 0 O CoO U)H C) 4-44 Ui u 0 u 0 CO u u r 0 *H -4--) 0 LO0 wr 0 4 C 0- 0 0 a) C)) U4 >Da _ r O C-4 4-) 0 o 2 , C NO -4 -4u eC) U 0 a) 4-) C H o 0) rrH H-) C H .- i -4 0 4- U)4 0D r_$ O) M 0 H_ C) 0 3-4 C) p ri 0 004.0 u 0 a) 0 (a ~ ~ a) C)0 C a) CD 04 ) 4- -H c-)L 0H w4 4) 0-) iLO 4 -) uI ) S - CD( a)1 a) U) 0)-u C) 4-)) 4- N( oo 4- - 4J CD~ (::) C o C - D M N -] * N . CD . D CD (N Hi u 0 CD C LO) LO) CD CD a D* HH LC) 0 a a D a) 4 a _1 a ~a~ N (N (N _j p ~ CC) 1-1 C C)M Lf) 1-1 aD C D C D a) CD U) -I a) 1 U) Lf C) a a C a) 4-) Lf) CCD CD 1 0 aa ( 4-) H z U) 0 04 a) 4- CD M 1- aD-I C (Y: M ) ~ j C- I Lf) . aD aD CD a a) u 4-) .H r-IL 0 U) .~ 0 u (N (N aa CD a) 0 'll U) CD U) CD a) 4) Q0 co~ co Hi ri-a U 4-) UU a) a) 4 a) 4-) 4-) la a) 4-) 4-) 4a) a) a) 4) 0 4 a) U 0Ua 0-) -' a) U) U) r_: 4-) a) 4-) a) 0 0 0 0 0 4-) -H - H -H -H U) 4-) 4-) 4-) 4-) 4-) 4-) 00 .r-4 x x 0~ u - F-H 41 -H F-H F-H WO 2014/022944 - 20 - PCT/CL2013/000047 As can be seen, in all cases the method of the present invention provides a cast steel with predominantly bainite structure and with higher Brinell hardness. 5 As can be seen in Fig. 6, the profile of Brinell hardness evaluated from the surface of the component toward its interior, to a depth of 13 inches (33.0 cm), remains practically constant. In contrast, the Cr-Mo pearlitic steel shows a considerable decrease in 10 hardness through its cross section. The foregoing description deals with the aims and advantages of the present invention. It must be understood that various embodiments of this invention 15 may be implemented and that all the subject matter disclosed here must be interpreted as being for purposes of illustration and is not limiting in any way.

Claims

1. A method for producing cast steel having high wear resistance, with predominantly bainite 5 microstructure and a suitable balance of toughness and hardness for mining applications such as grinding, crushing and all those applications that require large components with high resistance to wear by abrasion and impact, characterized in that 10 the chemical composition used, expressed in percentage by weight, comprises at least: - 0.30-0.40% w/w C; - 0.50-1.30% w/w Si; - 0.60-1.40% w/w Mn; 15 - 2.30-3.20% w/w Cr; - 0.00-1.00% w/w Ni; - 0.25-0.70% w/w Mo; - 0.00-0.50% w/w Cu; - 0.00-0.10% w/w Al; 20 - 0.00-0.10% w/w Ti; - 0.00-0.10% w/w Zr; - less than 0.050% w/w P; - less than 0.050% w/w S; - less than 0.030% w/w N; 25 - the remainder is iron; where the method comprises: a) completely melting the steel of the aforementioned composition; b) normalizing heat treatment at a temperature 30 between 950 and 1050'C, for a time of between 3 and 10 hours; followed by cooling from the normalizing temperature to a temperature between 500 and 80'C, at a rate in the range from 0.05 to 0.5 0 C/s; 35 c) annealing heat treatment at a temperature in the range from 450 to 630 0 C, for a time of between 3 and 10 hours. WO 2014/022944 - 22 - PCT/CL2013/000047

2. The method as claimed in claim 1, characterized in that the percentage by weight of chromium in the chemical composition of the steel is preferably 2.40-3.00% w/w. 5

3. The method as claimed in claims 1 or 2, characterized in that the chemical composition of the steel further comprises less than 0.050% w/w of niobium. 10

4. The method as claimed in any one of the preceding claims, characterized in that the chemical composition of the steel further comprises boron in the range 0.0005-0.005% w/w. 15

5. The method as claimed in any one of the preceding claims, characterized in that the chemical composition of the steel further comprises rare earths in the range 0.015-0.080% w/w. 20

6. The method as claimed in claim 5, characterized in that the rare earths correspond to commercial mixtures of cerium and lanthanum. 25

7. The method as claimed in any one of the preceding claims, characterized in that the chemical composition of the steel further comprises residual contents of tungsten, vanadium, tin, antimony, lead and zinc of less than 0.020% w/w. 30

8. The method as claimed in any one of the preceding claims, characterized in that the melting step (a) is carried out in an arc furnace. 35

9. The method as claimed in claim 8, characterized in that the arc furnace has a basic refractory or an acid refractory. WO 2014/022944 - 23 - PCT/CL2013/000047

10. The method as claimed in any one of claims 1 to 7, characterized in that the melting step is carried out in an induction furnace. 5

11. The method as claimed in claim 10, characterized in that the melting step (a) is carried out at a maximum temperature of 1700 C.

12. The method as claimed in any one of the preceding 10 claims, characterized in that the cooling in the normalizing heat treatment step (b) is carried out until a temperature of between 500 and 150'C is reached. 15

13. The method as claimed in any one of the preceding claims, characterized in that the cooling in the normalizing heat treatment step (b) is carried out in still air. 20

14. The method as claimed in any one of claims 1 to 12, characterized in that the cooling in the normalizing heat treatment step (b) is carried out in direct or indirect forced air. 25

15. The method as claimed in any one of claims 1 to 12, characterized in that the cooling in the normalizing heat treatment step (b) is carried out by a sequence of substeps in still air and in indirect forced air. 30

16. Cast steel having high wear resistance, with predominantly bainite microstructure and a suitable balance of toughness and hardness for mining applications such as grinding, crushing and 35 all those applications that require large components with high resistance to wear by abrasion and impact, characterized in that it is produced by the method as claimed in any one of WO 2014/022944 - 24 - PCT/CL2013/000047 claims 1 to 15.

17. Cast steel having high wear resistance and a suitable balance of toughness and hardness for 5 mining applications such as grinding, crushing and all those applications that require large components with high resistance to wear by abrasion and impact, characterized in that it comprises at least: 10 - 0.30-0.40% w/w C; - 0.50-1.30% w/w Si; - 0.60-1.40% w/w Mn; - 2.30-3.20% w/w Cr; - 0.00-1.00% w/w Ni; 15 - 0.25-0.70% w/w Mo; - 0.00-0.50% w/w Cu; - 0.00-0.10% w/w Al; - 0.00-0.10% w/w Ti; - 0.00-0.10% w/w Zr; 20 - less than 0.050% w/w P; - less than 0.050% w/w S; - less than 0.030% w/w N; and - the remainder is iron; and in that said steel has a predominantly bainite 25 structure.

18. The cast steel as claimed in claim 17, characterized in that the percentage by weight of chromium in the chemical composition of the steel 30 is preferably 2.40-3.00% w/w.

19. The cast steel as claimed in either of claims 17 or 18, characterized in that the chemical composition of the steel further comprises less 35 than 0.050% w/w of niobium.

20. The cast steel as claimed in any one of claims 17 to 19, characterized in that the chemical WO 2014/022944 - 25 - PCT/CL2013/000047 composition of the steel further comprises boron in the range 0.0005-0.005% w/w.

21. The cast steel as claimed in any one of claims 17 5 to 20, characterized in that the chemical composition of the steel further comprises rare earths in the range 0.015-0.080% w/w.

22. The cast steel as claimed in claim 21, 10 characterized in that the rare earths correspond to commercial mixtures of cerium and lanthanum.

23. The cast steel as claimed in any one of claims 17 to 22, characterized in that the chemical 15 composition of the steel further comprises residual contents of tungsten, vanadium, tin, antimony, lead and zinc of less than 0.020% w/w.