Low-alloy steel

Abstract

Provided is a low-alloy steel having a chemical composition comprising, in terms of % by mass, 0.6 – 0.8 C, 0.4 – 0.6 Si, 0.8 – 1.3 Mn, 2.0 – 3.0 Cr, 0.45 – 0.20 Ni, 0.45 – 0.65 Mo, 0.001 – 0.045 Ti, 0.015 – 0.05 Ce, 0.0002 – 0.007 B, and at least one of the compounds, in terms of % by mass, selected from 0.10 – 0.45 V, 0.15 – 0.50 W, and / or 0.05 – 0.35 Nb, and the remainder being of iron (Fe) and inevitable impurities.

Description

[0001] Low-alloy Steel

[0002] Field of invention

[0003] The present invention relates to a low-alloy steel having a specific chemical composition, a method of manufacturing a low-alloy steel and a low-alloy steel obtainable by inventive method. The invention further relates to an article comprising the inventive low-alloy steel and the use of the low-alloy steel according to the invention for preparing an article.

[0004] Background

[0005] High-strength low-alloy (HSLA) steels, or micro alloyed steels, are designed to provide better mechanical properties and / or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition (HSLA steels have yield strengths greater than 275 MPa, or 40 psi). The chemical composition of a specific HSLA steel may vary for different product thicknesses to meet mechanical property requirements. A steel is characterized as being "low-alloy" if it comprises less than 8% of the alloying elements such as chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium. These are all used in various combinations. In a multicomponent alloy system, the alloying element can be found in (1) free state, (2) as intermetallic compound (3) as oxides, sulfides, and other non-metallic inclusions (4) as a solution in iron. Based on this we can categorize the elements into two groups.

[0006] Group 1: Elements that do not form carbides (e.g., Ni, Si, Co, Al, Cu and N)

[0007] Group 2: Elements that form stable carbides (e.g., Cr, Mn, Mo, W, V, Ti, Zr, and Nb)

[0008] Precipitation and distribution of carbides in the microstructure of the low-alloy steel can be controlled and manipulated by changing the heat treatment, especially by the cooling process during heat treatment.

[0009] Typically, the mechanical properties of low-alloy steels are a tradeoff between hardness and toughness, and HSLA steels are therefore often manufactured on the basis of minimum mechanical properties, with the specific alloy content left to the discretion of the steel producer.

[0010] In applications that require high hardness and also a high toughness of the HSLA, it is necessary to make a compromise on one of the properties.

[0011] Traditional austenitic manganese steel (AMS) can offer both high hardness and toughness, however this can be only when the material is subjected to impact conditions. In absence of impact / work hardening the alloy suffers higher abrasion rates. This often is addressed by hard facing when AMS is used under abrasion condition. However, depending on the shape of a molded steel element, it is not always possible to work the steel to obtain the desired hardness and toughness. Furthermore, a worked steel element may obtain a high hardness and toughness towards the surface, but lower hardness and toughness towards the center of the steel element. This is not desirable for wear elements, where the surface is worn over time.

[0012] It would be desirable to provide a low-alloy steel having both improved hardness and toughness by optimizing the chemistry and heat treatment of the steel to avoid working the steel. Summary of the invention

[0013] With this background, it is therefore an object of the present invention to provide a low-alloy steel by which it is possible to mitigate some of the drawbacks of the prior art.

[0014] In a first aspect of the invention, these and further objects are obtained by a low-alloy steel having a chemical composition comprising, in terms of % by mass: and at least one of the compounds, in terms of % by mass, selected from and the remainder being of iron (Fe) and inevitable impurities.

[0015] It has been found that the above low-alloy steel composition promotes the formation of small carbides which are finely and uniformly embedded within a continuous ferrite matrix. It has been found that the hardness and toughness of this above composition is higher compared to similar low-alloy steel compositions. The hardness and toughness may be optimized and improved using different heat treatments.

[0016] Because of the characteristic of forming small and finely distributed carbides it is believed that the continuous ferrite matrix embedding the carbides improves the toughness of the low-alloy steel without lowering the hardness. This characteristic is especially applicable in applications where a high toughness is required, since a high toughness can be achieved while simultaneously achieving a high hardness, compared to state of the art low-alloy steel compositions.

[0017] Inevitable impurities may be other alloys added to the low-alloy steel in connection with the manufacturing process. Inevitable impurities according to the invention are elements being present in the composition in an amount of below 0.1 % by mass. Inevitable impurities according to the present invention are for example cobalt, lead, antimony, sulfur, phosphorus, copper, tin, zinc, arsenic, bismuth, tantalum, calcium, zirconium, lanthanum, selenium, aluminum and nitrogen. Typical impurity elements and their concentrations may be found in the later described examples.

[0018] In one or more embodiments the microstructure of at least a portion of the low-alloy steel comprises precipitated carbides. In particular, the microstructure may be precipitated carbides in ferrite.

[0019] In one or more embodiments the microstructure of at least a portion of the low-alloy steel is tempered martensite or bainite. Preferably the predominant microstructure of the low-alloy steel is tempered martensite or bainite.

[0020] In tempered martensite the carbides may be extremely small and uniformly dispersed particles embedded within a continuous ferrite matrix. The continuous matrix in combination with the small and uniformly distributed particles provides a hard and strong material, which may be almost as hard and strong as martensite but with substantially enhanced ductility and toughness.

[0021] The microstructure of bainite is similar to the one of tempered martensite but has a fine non-lamellar structure commonly consisting of carbides and dislocation-rich ferrite. The large density of dislocations in the ferrite in combination with the fine size of the bainite platelets, provides the good properties. The hardness of a low-alloy steel with a microstructure of bainite is typically lower than a microstructure of martensite.

[0022] Whereas martensite is formed by rapid cooling, bainite is formed by moderate cooling, e.g., by austempering.

[0023] In one or more embodiments the microstructure is a tempered martensite with finely dispersed carbides particles.

[0024] In one or more embodiments the precipitated carbides comprise one or more of niobium, tungsten, chromium, boron, and / or molybdenum carbides.

[0025] In one or more embodiments the precipitated carbides comprise a combination of niobium, tungsten, chromium, boron, and / or molybdenum carbides. Cementite and other iron carbides may also be present.

[0026] Preferably the precipitated carbides are of one or more of the types selected from the list of MC, M2C, M6C, M7C3, and M23C6, where C is a carbon atom and M is a Metal ion.

[0027] The elements Vanadium, Tungsten or Niobium are added to form carbides in the alloy. By adding Vanadium, Tungsten, and / or Niobium in small amounts together with Cerium and Boron the alloy becomes much harder and tougher than other low-alloy steel compositions. Without being bound by any theory it is believed that by providing Cerium and Boron to the alloy in the specified amounts, a finer grain structure is provided and also precipitation of the carbides inside the grains is provided. Thus, by having the carbides better distributed through the alloy, the alloy is bound together. Hence, it is possible to obtain a low-alloy steel having very high toughness and hardness by adding only low amounts of the carbide forming alloys. In one or more embodiments the low-alloy steel comprises at least two of the elements listed below in the specified amounts: or the low alloy steel comprises the below elements in the specified amounts:

[0028] In one or more embodiments the low-alloy steel has a Carbon content of between 0.6 and 0.8 w / w%, preferably between 0.65 and 0.75 w / w%, more preferably between 0.66 and 0.72 w / w%, such as 0.66 w / w%, 0.67 w / w%, 0.68 w / w%, 0.69 w / w%, 0.70 w / w%, 0.71 w / w% or 0.72 w / w%.

[0029] In one or more embodiments the low-alloy steel has a Silicon content of between 0.40 and 0.60 w / w%, preferably between 0.45 and 0.58 w / w%, such as 0.45 w / w%, 0.46 w / w%, 0.47 w / w%, 0.48 w / w%, 0.49 w / w%, 0.50 w / w%, 0.51 w / w%, 0.52 w / w%, 0.53 w / w%, 0.54 w / w%, 0.55 w / w%, 0.56 w / w%, 0.57 w / w%, or 0.58 w / w%.

[0030] In one or more embodiments the low-alloy steel has a Manganese content of between 0.80 w / w% and 1.30 w / w%, preferably between 0.85 w / w% and 1.25 w / w%, more preferably between 0.90 w / w% and 1.20 w / w%, more preferably between 0.95 w / w% and 1.18 w / w%.

[0031] In one or more embodiments the low-alloy steel has a Chromium content of between 2.0 w / w% and 3.0 w / w%, preferably between 2.10 w / w% and 2.90 w / w%, more preferably between 2.15 w / w% and 2.80 w / w%, more preferably between 2.20 w / w% and 2.70 w / w%, more preferably between 2.25 w / w% and 2.70 w / w%.

[0032] In one or more embodiments the low-alloy steel has a Nickel content of between 0.045 w / w% and 0.20 w / w%, preferably between 0.050 w / w% and 0.175 w / w%, more preferably between 0.050 w / w% and 0.165 w / w%.

[0033] In one or more embodiments the low-alloy steel has a Molybdenum content of between 0.45 w / w% and 0.65 w / w%, preferably between 0.475 w / w% and 0.60 w / w%, more preferably between 0.50 w / w% and 0.575 w / w%. In one or more embodiments the low-alloy steel has a Titanium content of between 0.001w / w% and 0.045 w / w%, preferably between 0.002 w / w% and 0.04 w / w%, more preferably between 0.002 w / w% and 0.037 w / w%.

[0034] In one or more embodiments the low-alloy steel has a Cerium content of between 0.015 w / w% and 0.05 w / w%, preferably between 0.018 w / w% and 0.05 w / w%, more preferably between 0.020 w / w% and 0.046 w / w%.

[0035] In one or more embodiments the low-alloy steel has a Boron content of between 0.0002 w / w% and 0.007 w / w%, preferably between 0.001 w / w% and 0.006 w / w%, more preferably between 0.0015 w / w% and 0.005 w / w%, more preferably between 0.0020 w / w% and 0.0045 w / w%, more preferably between 0.0021 w / w% and 0.004, more preferably between 0.0021 w / w% and 0.0038 w / w%

[0036] In one or more embodiments the low-alloy steel has a Vanadium content of between 0.10 w / w% and 0.45 w / w%, preferably between 0.20 w / w% and 0.4 w / w%, more preferably between 0.25 w / w% and 0.35 w / w%, more preferably between 0.27 w / w% and 0.34 w / w%, more preferably between 0.28 w / w% and 0.33 w / w%, such as 0.28 w / w%, 0.29 w / w%, 0.30 w / w%, 0.31 w / w%, 0.32 w / w%, or 0.33 w / w%.

[0037] In one or more embodiments the low-alloy steel has a Tungsten content of between 0.15 w / w% and 0.50 w / w%, preferably between 0.20 w / w% and 0.45 w / w%, more preferably between 0.25 w / w% and 0.40 w / w%, more preferably between 0.30 w / w% and 0.36 w / w%, more preferably between 0.31 w / w% and 0.35, such as 0.31 w / w%, 0.32 w / w%, 0.33 w / w%, 0.34 w / w%, or 0.35 w / w%.

[0038] In one or more embodiments the low-alloy steel has a Niobium content of between 0.05 w / w% and 0.35 w / w%, preferably between 0.1 w / w% and 0.3 w / w%, more preferably between 0.15 w / w% and 0.25 w / w%, more preferably between 0.18 w / w% and 0.23 w / w, such as 0.18 w / w%, 0.19 w / w%, 0.20 w / w%, 0.21 w / w%, 0.22 w / w% or 0.23 w / w%.

[0039] In one or more embodiments the low-alloy steel comprises at least two of the elements Vanadium, Tungsten and / or Niobium in the concentrations specified above.

[0040] In one or more embodiments the low-alloy steel comprises Vanadium, Tungsten and Niobium in the concentrations specified above.

[0041] In a preferred embodiment the low-alloy steel a comprises at least two of the elements listed below in the specified amounts: or the low alloy steel comprises the below elements in the specified amounts:

[0042] In one or more embodiments the low-alloy steel has a Sulfur content of below 0.025 w / w%, and preferably below 0.020 w / w%.

[0043] In one or more embodiments the low-alloy steel has a Phosphorous content of below 0.025 w / w%, and preferably below 0.020 w / w%.

[0044] Sulfur and Phosphorous are impurities. It is preferable to keep the content of Sulfur and Phosphorous low.

[0045] In one or more embodiments the low-alloy steel comprises cobalt, lead, antimony, sulfur, phosphorus, copper, tin, zinc, arsenic, bismuth, tantalum, calcium, zirconium, lanthanum, selenium, aluminum and / or nitrogen wherein each element, if present, is present in an amount of below 0.1 % by mass.

[0046] In one or more embodiments the low-alloy steel is heat treated after casting, to form a microstructure which is martensite, tempered martensite, and / or bainite.

[0047] Preferably the predominant microstructure of the heat treated low-alloy steel is tempered martensite or bainite.

[0048] By providing a low-alloy steel comprising Vanadium, Tungsten and / or Niobium it is possible to manufacture a steel having a Brinell Hardness Number (BHN) after heat treatment of 380 BHN or more, preferably 400 BHN or more, more preferably 420 BHN or more, even more preferably 430 BHN or more, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, and simultaneously obtain a Toughness of 25 J or more, preferably 28 J or more, more preferably of 29 J or more, even more preferably of 33 J or more, and most preferably of 45 J or more in un-notched charpy impact test, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0049] The specific hardness and toughness will depend on the heat treatment.

[0050] In one or more embodiments the low-alloy steel has a Brinell Hardness Number after casting above 400 BHN, preferably above 500 BHN, more preferably above 550 BHN, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10.

[0051] In one or more embodiments the low-alloy steel has a Toughness of above 15 J in un-notched charpy impact test, preferably above 20 J in unnotched charpy impact test, preferably above 30 J in unnotched charpy impact test, preferably above 50 J in unnotched charpy impact test, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10.

[0052] In one or more embodiments the low-alloy steel has a Hardness after tempering above 45 HRC, preferably above 50 HRC, more preferably above 55 HRC, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10. In one or more embodiments the low-alloy steel has a Hardness after heat treatment above 370 BHN, preferably above 380 BHN, more preferably above 390 BHN, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0053] In one or more embodiments the low-alloy steel has a Toughness of above 20 J in un-notched charpy impact test, preferably above 25 J in unnotched charpy impact test, more preferably above 28 J in unnotched charpy impact test, preferably above 33 J in unnotched charpy impact test, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0054] In one or more embodiments the low-alloy steel has a Hardness after casting of at least 450 BHN, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, and a Toughness of at least 10 J in un-notched condition, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, preferably a Hardness of at least 500 BHN and a Toughness of at least 15 J in un-notched condition, most preferably a Hardness of at least 550 BHN and a Toughness of at least 16 J in un-notched condition.

[0055] In one or more embodiments the low-alloy steel has a Hardness after heat treatment of at least 380 BHN, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, and a Toughness of at least 20 J in un-notched condition, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, preferably a Hardness of at least 385 BHN and a Toughness of at least 25 J in un-notched condition, and more preferably a Hardness of at least 395 BHN and a Toughness of at least 28 J in un-notched condition.

[0056] The low-alloy steel of the composition above and having a Vanadium content of between 0.10 w / w% and 0.45 w / w% may obtain a Brinell Hardness Number after heat treatment of between 390 BHN to 420 BHN, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, and simultaneously having a Toughness of between 40 J and 55 J, preferably having a Toughness between 42 J and 52 J in un-notched charpy impact test, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0057] The low-alloy steel of the composition above and having a Tungsten content of between 0.15 w / w% and 0.50 w / w% may obtain a Brinell Hardness Number after heat treatment of between 400 BHN to 450 BHN, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, and simultaneously having a Toughness of between 20 J and 30 J in un-notched charpy impact test, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0058] The low-alloy steel of the composition above and having a Niobium content of between 0.05 w / w% and 0.35 w / w% may obtain a Brinell Hardness Number after heat treatment of between 385 BHN to 410 BHN, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10, and simultaneously having a Toughness of between 30 J and 40 J in un-notched charpy impact test, measured on specimen having a dimension of 300 mm x 300 mm x 300 mm and measured according to ASTM A370 / E10.

[0059] By adding Niobium, Tungsten and Vanadium in combination may provide an alloy having a combination of the hardness and toughness described.

[0060] The low-alloy steel of the composition above and having a Niobium content of between 0.05 w / w% and 0.35 w / w%, a Tungsten content of between 0.15 w / w% and 0.50 w / w%, and a Vanadium content of between 0.10 w / w% and 0.45 w / w% may obtain a Brinell Hardness Number after casting of between 450 BHN to 650 BHN, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, and simultaneously having a Toughness of between 55 J and 105 J in un-notched charpy impact test, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10.

[0061] The low-alloy steel of the composition above and having a Niobium content of between 0.05 w / w% and 0.35 w / w%, a Tungsten content of between 0.15 w / w% and 0.50 w / w%, and a Vanadium content of between 0.10 w / w% and 0.45 w / w% may obtain a Brinell Hardness Number after tempering of between 55 HRC to 70 HRC, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, and simultaneously having a Toughness of between 55 J and 105 J in un-notched charpy impact test, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10.

[0062] Any value of Hardness referred to, is found by using the Brinell hardness tester measuring according to ASTM A370 / E10.

[0063] Any value of Toughness referred to, is found by using the Charpy impact testing in un-notched condition at room temperature measuring according to ASTM A370 / E10.

[0064] According to another aspect, the invention relates to a wear part made of the low-alloy steel as defined in the description above and in the claims. The wear part may be digging elements such as shovels, milling bits and teeth, and crushing and milling elements such as rollers, cones, liners. By providing a wear part made of the low-alloy steel it is possible to provide a wear part having high hardness and toughness without working the surface of the low-alloy steel. Furthermore, the low-alloy steel allows for more uniform hardness and toughness properties compared to worked low-alloy steels.

[0065] In particular, the inventive alloy finds its application within heavy duty wear applications, such as in mining, grinding, and milling.

[0066] In particular, the inventive alloy may be used to manufacture wear lining, such as linings to chutes, mills, grinders.

[0067] According to another aspect, the invention relates to a method of manufacturing a low-alloy steel comprising, preferably consisting of, the steps of: providing a cast low-alloy steel part having a composition of: and further comprising at least one of the compounds selected from and the remainder being of iron (Fe) and inevitable impurities, heat treating the low-alloy steel to form dispersed carbides.

[0068] In one or more embodiments the heat treatment comprises a normalizing step, a quenching step and / or a tempering step.

[0069] Preferably the low-alloy steel composition is heat treated such that at least a portion of the low-alloy steel having a bainitic structure, a martensite structure and / or a tempered martensite structure.

[0070] In one or more embodiments the heat treatment comprises a normalizing step comprising the steps of:

[0071] - heating the low-alloy steel at a rate not greater than 100°C / hr, to an austenitic temperature range,

[0072] - maintaining the low-alloy steel at the austenitic temperature range for at least 1 hr / pr 25mm of thickness,

[0073] - slow cooling the low-alloy steel to room temperature, preferably by atmospheric air cooling. The cooling rates in slow cooling may be in the range of 3°C / min to 8°C / min, preferably 4°C / min to 6°C / min.

[0074] Depending on the specific composition of the low-alloy steel the austenitic temperature range may vary. For formalization it is typically suitable to heat the low-alloy steel to a temperature greater than 900°C, preferably to 920°C - 960°C, e.g. 930°C, 940°C or 950°C.

[0075] Preferably the low-alloy steel is heated at a rate not greater than 90°C / hr, more preferably not greater than 80°C / hr. Even if the thickness of the low-alloy steel is less than 50 mm, it is preferred to maintain the temperature at the austenitic temperature range for at least 2 hours / pr 25mm of thickness. Preferably for at least 5 hours / pr 25mm of thickness.

[0076] Normalizing is a heat-treatment process applied to refine grain structure and create a homogenous microstructure. This treatment is performed to relieve internal stresses or to resolve inconsistent fiber structure which developed during the casting process.

[0077] In one or more embodiments the heat treatment comprises a tempering step comprising the steps of:

[0078] - heating the low-alloy steel to an elevated temperature below an austenitic temperature range, preferably to below 650°C, more preferably to 500°C to 650°C, even more preferably to below 500°C, and even more preferably to 200°C to 450°C, e.g. 220°C, 250°C or 300°C.

[0079] - maintaining the low-alloy steel at the elevated temperature below the austenitic temperature range for at least 1 hr / pr 25 mm of thickness;

[0080] - cooling the low-alloy steel to room temperature, preferably by moderate cooling such as forced air cooling. The cooling rates may be in the range of 6°C / min to 12°C / min, preferably 7°C / min to 10°C / min.

[0081] It may be preferred to heat the low-alloy to a temperature in the range of 200°C to 250°C such as for example 200°C, 210°C, 220°C, 230°C, 240°C or 250°C, to avoid the risk of temper embrittlement. As the tempering is conducted at higher temperatures there is a risk of softening the low-alloy steel.

[0082] Tempering is a heat-treatment process in which cooling is applied at a proper speed. Tempering is especially applied performing a quench-hardening process, the material is heated again, then tempering is applied. Quenching is applied to adjust hardness, to add toughness, and to relieve internal stress. There are two types of tempering — one is high-temperature tempering, and the other is low-temperature tempering. Applying the tempering at higher temperature, more toughness is obtained, although the hardness decreases. For thermal refining, high-temperature tempering is performed. For induction hardening or carburizing, the required tempering performed after surface-hardening treatment is low-temperature tempering. For the specific low-alloy steel composition a quenching step would cause the formation of martensite structure which is quite brittle. The tempering heat treatment allows, by diffusional processes, the formation of tempered martensite.

[0083] In one or more embodiments the heat treatment comprises a quenching step comprising the steps of:

[0084] - heating the low-alloy steel at a rate not greater than 100°C / hr, to the austenitic temperature range, preferably to 900°C - 930°C, such as e.g. 910°C or 920°C,

[0085] - maintaining the temperature for at least 1 hr / pr 25 mm of thickness; and

[0086] - cooling the low-alloy steel by quench cooling, preferably by liquid quenching, such as water quenching, or forced air cooling with mist.

[0087] Preferably the heating of the low-alloy steel is provided at a rate not greater than 90°C, preferably not greater than 80°C / hr. Quenching is a heat-treatment process of steel in which rapid cooling is applied after heating to high temperature. There are several types of quenching in accordance with cooling conditions: water quenching, oil quenching, and vacuum quenching.

[0088] If the cooling rate is not high enough to convert austenite to martensite, bainite is typically formed.

[0089] If a microstructure of bainite is desirable the low-alloy steel may be austempered instead of quenched. Austempering is similar to quenching, but instead of quench cooling to temperatures below 200°C, the low- alloy steel part is quench cooled from the austenitizing temperature range to 230°C to 400°C, and thereafter maintaining the temperature substantially constant to allow isothermal transformation.

[0090] By moderately cooling the low-alloy steel from the austenitic temperature range a microstructure which is predominantly (80-90%) carbides of niobium, tungsten, chromium, boron, and molybdenum, in bainitic structure is obtained. The remaining being 10-20 % martensite.

[0091] By fast to moderately cooling the low-alloy steel from the austenitic temperature range to ambient temperature a microstructure may be provided which is finely dispersed carbides of niobium, tungsten, chromium, boron, and molybdenum, in bainitic and martensitic structure.

[0092] By quench cooling the low-alloy steel from the austenitic temperature range to ambient temperature a microstructure may be provided which is predominantly (>95%) finely dispersed carbides of niobium, tungsten, chromium, boron, and moly, in martensitic structure.

[0093] The low-alloy steel may be manufactured by the following method:

[0094] Providing a molten steel mixture having the above composition;

[0095] Deoxidizing the molten steel composition to remove oxygen;

[0096] Heating the melt to a temperature above the liquidous temperature;

[0097] Casting the molten steel composition to a desired shape;

[0098] Cooling the cast steel such that substantially no thermal stresses are induced during cooling to an ambient temperature;

[0099] Optionally cleaning and / or shot blasting the cast steel.

[0100] The liquidous temperature may be from in the range of from 1400 to 1600°C, e.g. 1475°C, depending on the composition of the alloy.

[0101] In one or more embodiments the Cerium and Boron may be added after the deoxidation step. Cerium and Boron may be added as ferro-alloy or as substantially pure Cerium and Boron.

[0102] The austenitizing temperature may be from 850°C to 1000°C depending on the composition of the alloy.

[0103] In another aspect the present invention relates to a low-alloy steel obtainable by the method according to the invention. Preferably the low-alloy steel is a low-alloy steel as described above and in the claims.

[0104] In a further aspect the present invention relates to an article comprising, preferably consisting of, the low-alloy steel according to the present invention, wherein the article preferably is a wear part.

[0105] A wear part may be digging elements such as shovels, milling bits and teeth, and crushing and milling elements such as rollers, cones, liners. Preferably the wear part is selected from the group of shovels, milling bits and teeth, rollers, cones or liners.

[0106] The wear part can also be a gyratory crusher wear part, a cone crusher wear part, a sizer wear part, a mill wear part for roller mills, ball mills, SAG mills, hammer mills, or a HPGR wear parts.

[0107] A wear part made of the low-alloy steel according to the invention has a high hardness and toughness without working the surface of the low-alloy steel. Furthermore, the low-alloy steel allows for more uniform hardness and toughness properties compared to worked low-alloy steels.

[0108] In another aspect the present invention relates to the use of the low-alloy steel according to the invention for preparing an article, wherein the article preferably is a wear part.

[0109] The wear part can be any wear part as described above.

[0110] According to another aspect the present invention relates to the use of the low-alloy steel according to the invention in heavy duty wear applications, such as in mining, grinding, and milling.

[0111] In another aspect the present invention relates to the use of the low-alloy steel according to the invention for manufacturing wear lining, such as linings to chutes, mills, grinders.

[0112] Further presently preferred embodiments and further advantages will be apparent from the following detailed description, examples, and the appended dependent claims.

[0113] Brief description of the drawings

[0114] The invention will be described in more details below by means of non-limiting examples of presently preferred embodiments and with reference to the figures, in which:

[0115] Fig. 1 shows a comparison in mechanical properties (Brinell Hardness [BHN] and Toughness [J]) between a standard alloy and three different alloys according to the invention measured on samples having a dimension of 300 mm x 300 mm x 300 mm. Mode for carrying out the invention

[0116] Three different alloy compositions were manufactured in samples "New Alloy A", "New Alloy B", "New Alloy C" also referred to as "Alloy A", "Alloy B", "Alloy C".

[0117] The Alloy "Baseline Alloy" corresponds to a reference Cr-Mo micro alloy having a composition as the other alloys but without any Niobium, Vanadium and Tungsten.

[0118] The cast test samples were tested for hardness and impact values and compared to the common steel alloy to understand the effect of alloying elements on the properties.

[0119] The alloys were manufactured according to the table below.

[0120] Table 1: Mass ranges of inventive Alloys A to C

[0121] Each of Alloy A, Alloy B, and Alloy C was prepared using the following method.

[0122] Steel scraps, return and ferro alloys were provided into a 50 kg inductotherm induction furnace. The metal was heated until it reached the liquidous temperature. Samples were taken and chilled, and the chemistry was measured using optical spectrometer. The composition was adjusted accordingly, and samples were taken until the alloy reached a desired composition which can be seen in the Table 2 below. Table 2: Determined compositions of inventive alloys A, B and C

[0123] After adjusting the alloys to their final chemistry, the melt was deoxidized by adding aluminum and SiZr.

[0124] For deoxidation 1.3kg / ton of SiZr and 0.5 kg / ton aluminum were added to the melt stream as the metal was transferred from the furnace to the pouring ladle. The residual aluminum after deoxidation was less than 0.06% by mass. The pouring temperature of the liquid metal was around 1485-1520°C and was checked using an immersion pyrometer. After pouring into the test coupon molds, the molds were cooled until below 300 °C. The cast samples were then removed from the molds and subsequently cleaned by shot blasting and fettled to prepare for heat treatment.

[0125] From each of the alloys at least three samples were made as per Japanese Industrial Standards (JIS) G 0307.

[0126] Hardness value of the casted samples were measured according to ASTM A370 / E10 and thereafter the casted samples were subjected to different heat treatments, where the cast samples were normalized, tempered and then air quenched and tempered.

[0127] Normalization was achieved by heating the samples at a rate not greater than 80 °C / hr to 920-960 °C and held for 1 hr / 25mm of thickness and air cooled to room temperature.

[0128] Tempering was performed immediately by heating the samples at a rate of 80 °C / hr to 200 - 225°C and held for 1 hr / 25mm of sample thickness, followed by still air cooling to room temperature.

[0129] The samples were then quenched and tempered to normalize the grains and homogenize the steel to improve hardenability.

[0130] The quenching step was achieved by heating the samples at a rate of 80°C / hr to 900 - 930°C and held for 1 hr / 25mm of thickness. The samples were then quench cooled to room temperature using forced air.

[0131] Tempering was subsequently achieved by heating the samples at a rate of 80°C / hr to 200 - 225°C and held for 1 hr / 25mm of sample thickness, followed by still air cooling to room temperature.

[0132] Alloy D

[0133] Based on the manufacturing of Alloy A, Alloy B, and Alloy C an alloy composition "Alloy D" was manufactured and tested. Alloy D comprising all three of the elements Vanadium, Tungsten and Niobium in the amounts as specified in Table 3. The remaining elements being within the range specified for Alloy A, Alloy B, and Alloy C as shown in Table 1.

[0134] Table 3: Mass ranges of V, W and Nb in Alloy D

[0135] Test results

[0136] All mechanical testing was performed as per ASTM A370 / E10.

[0137] Tensile test samples were taken 10mm below the surface to eliminate any surface effects of heat treatment. Charpy impact testing were performed in un-notched condition at room temperature; a Brinell hardness machine or an Equotip™ machine were used to measure hardness in Rockwell on the broken impact test specimens, the same sample was used for metallography analysis.

[0138] Table 4: Test results of mechanical testing of specimen of Alloys A - D and a Baseline Alloy prepared according to Japanese Industrial Standards (JIS) G 0307; values are averaged from three test samples.

[0139] It was found that for the smaller samples, cf. Table 4, all inventive alloys showed higher hardness values in comparison to the Baseline Alloy. Alloy D after tempering had a high toughness value compared to Alloy A, Alloy B, Alloy C, and the Baseline Alloy while maintaining a very high hardness.

[0140] The same method as described above was used to manufacture samples from Alloy A, B and C, having a dimension of 300 mm x 300 mm x 300 mm. These samples were heat treated as described above, but due to the greater thickness each of the heating and cooling steps took longer times.

[0141] Table 5: Test results of mechanical testing after heat treatment of specimen having a dimension of 300 mm x 300 mm x 300 mm; ; values are averaged from three test samples.

[0142] For those larger samples hardness as well as toughness of the inventive Alloys A, B and C were better in comparison to the Baseline Alloy (current Cr-Mo in Fig. 1). Fig. 1 as well as Table 5 also show that these samples have a lower hardness but greater toughness compared to the smaller samples, cf. Table 4.

Claims

Claims1. A low-alloy steel having a chemical composition comprising, in terms of % by mass:and at least one of the compounds, in terms of % by mass, selected fromand the remainder being of iron (Fe) and inevitable impurities.

2. The low-alloy steel according to claim 1, wherein the microstructure of at least a portion of the low- alloy steel is precipitated carbides in ferrite; and / or wherein the microstructure of at least a portion of the low-alloy steel is tempered martensite or bainite, preferably wherein the predominant microstructure of the low-alloy steel is tempered martensite or bainite.

3. The low-alloy steel according to any previous claim, wherein the microstructure of the low-alloy steel is a tempered martensite with finely dispersed carbides particles.

4. The low-alloy steel according to claims 2 to 3, wherein the precipitated carbides comprise one or more of niobium, tungsten, chromium, boron, and / or molybdenum carbides; or wherein the precipitated carbides comprise a combination of niobium, tungsten, chromium, boron, and molybdenum carbides, preferably also cementite and / or iron carbides are present.

5. The low-alloy steel according to any previous claim, wherein the low-alloy steel comprises at least two of the elements listed below in the specified amounts:or wherein the low alloy steel comprises the below elements in the specified amounts:

6. The low-alloy steel according to any previous claim, having a Brinell Hardness Number after casting above 400 BHN, preferably above 500 BHN, more preferably above 550 BHN, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, and / or having a Toughness of above 15 J in un-notched charpy impact test, preferably above 20 J in unnotched charpy impact test, preferably above 30 J in unnotched charpy impact test, preferably above 50 J in unnotched charpy impact test, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10.

7. The low-alloy steel according to any previous claim having a Hardness after casting of at least 450 BHN, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, and a Toughness of at least 10 J in un-notched condition, measured on specimen prepared according to Japanese Industrial Standards (JIS) G 0307 and measured according to ASTM A370 / E10, preferably a Hardness of at least 500 BHN and a Toughness of at least 15 J in un-notchedcondition, most preferably a Hardness of at least 550 BHN and a Toughness of at least 16 J in un-notched condition.

8. Method of manufacturing a low-alloy steel according to any previous claim, wherein the method comprises the steps of:- providing a cast low-alloy steel having a composition according to claims 1 to 7;- heat treating the low-alloy steel to form dispersed carbides.

9. The method according to claim 8, wherein heat treatment comprises a normalizing step, a quenching step and / or a tempering step.

10. The method according to claims 8 to 9, wherein the heat treatment comprises a normalizing step comprising the steps of:- heating the low-alloy steel at a rate not greater than 100°C / hr, preferably not greater than 80°C / hr, to an austenitic temperature range, preferably to greater than 900°C, more preferably to 920°C -960°C;- maintaining the low-alloy steel at the austenitic temperature range for at least 1 hr / pr 25mm of thickness, preferably for at least 2 hours / pr 25mm of thickness, more preferably for at least 5 hours / pr 25mm of thickness;- slow cooling the low-alloy steel, preferably in cooling rates in the range of 3°C / min to 8°C / min, to room temperature, preferably by atmospheric air cooling.

11. The method according to claims 8 to 10, wherein the heat treatment comprises a tempering step comprising the steps of:- heating the low-alloy steel to below an austenitic temperature range, preferably to below 650°C, and more preferably below 500°C and even more preferably to 200°C to 450°C;- maintaining the low-alloy steel at the temperature below the austenitic temperature range for at least 1 hr / pr 25 mm of thickness;- cooling the low-alloy steel, preferably in cooling rates in the range of 6°C / min to 12°C / min, to room temperature, preferably by forced air cooling.

12. The method according to claim 8 to 11, wherein the heat treatment comprises a quenching step comprising the steps of:- heating the low-alloy steel at a rate not greater than 100°C / hr, preferably not greater than 80°C / hr, to the austenitic temperature range, preferably to 900°C - 930°C;- maintaining the temperature for at least 1 hr / pr 25 mm of thickness; and- cooling the low-alloy steel by quenching, preferably by forced air cooling, still air cooling, or liquid quenching, such as water quenching or forced air cooling with mist, to form a martensite structure.

13. A low-alloy steel obtainable by the method according to claims 8 to 12, preferably the low-alloy steel is a steel according to claims 1 to 7.

14. An article comprising, preferably consisting of, the low-alloy steel according to claims 1 to 7 and 13, wherein the article preferably is a wear part.

15. Use of the low-alloy steel according to claims 1 to 7 and 13 for preparing an article, wherein the article preferably is a wear part.

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