MXPA00007198A - Steel powder for the preparation of sintered products - Google Patents

Abstract

<0.2, C<0.01 the balance being iron and, an amount of not more than 1%, inevitable impurities, at a pressure of at least 600 MPa and subjecting the compacted body to sintering at a temperature of at most 1220°C. The invention also concerns the annealed powder used in the method as well as the sintered products.

Description

POWDER STEEL FOR THE PREPARATION OF SINTERED PRODUCTS FIELD OF THE INVENTION The present invention relates to chromium alloy powder steel. Specifically, the invention relates to low oxygen and low carbon alloy steel powder which includes, in addition to iron and chromium, Mo and Mn, as well as preparations thereof. The invention also relates to a method for preparing sintered components from this powder, as well as the sintered components.

BACKGROUND OF THE INVENTION Recently, various techniques have been developed to strengthen materials for the manufacture of machine-made sintered parts produced from powdered steel of various alloys by powder metallurgy. In the patent of E.U.A. No. 4,266,974 and in European Patent No. 0 653 262, it is suggested for example the use of alloying elements or binders such as chromium, molybdenum and manganese in iron powders with low oxygen and carbon content. The base material for the powder in both patents is an atomized powder with water and annealing by reduction. The patent of E.U.A. it states that the most important step in obtaining a powder with a low content of oxygen and carbon is the annealing step, which preferably should be carried out under reduced pressure, specifically by vacuum induction heating. The patent of E.U.A. It also indicates that other methods of annealing by reduction have drawbacks or disadvantages that limit their installation on a commercial scale. For its part, the European patent does not describe annealing by reduction. The effective amounts of alloying elements in accordance with the patent of E.U.A. they are between 0.2 and 5.0% by weight of chromium, 0.1 and 7.0% by weight of molybdenum, and 0.35 and 1.50% by weight of manganese. The European patent for its part indicates that the effective amounts should be between 0.5 and 3% by weight of chromium, 0.1 and 2% by weight of molybdenum and at most 0.08% by weight of manganese. The object of the invention in accordance with the patent of E.U.A. is to provide a powder that meets the demands for compressibility and moldability or high dust plasticity, as well as good heat treatment properties, such as carburizing and hardenability, in the sintered body. A major disadvantage when using the invention described in the European patent application, is that the waste material or the waste can not be used because these residues normally include more than 0.08% by weight of manganese. In this context, the European patent application states that a specific treatment must be carried out to reduce the Mn content to a level that does not exceed 0.08% by weight. Another problem is that this patent does not say anything about the annealing by reduction and the possibility of obtaining a low content of oxygen and carbon in water-atomized powder steels, including elements sensitive to oxidation such as chromium and manganese. The only information in this regard appears in Example 1, which describes the need for a final reduction.

BRIEF DESCRIPTION OF THE INVENTION Broadly speaking, the present invention relates to chromium-based powder steel with low oxygen and carbon content, which includes between 2.5 and 3.5% by weight of chromium, 0.3 and 0.7% by weight of molybdenum and 0.09 and 0.3. % by weight of manganese. This composition allows the production of sintered components with excellent mechanical properties from a raw material atomized with water and annealed by reduction, low cost. Unexpectedly, it has been found that the sintered products prepared from the powder according to the invention are distinguished by a combination of high tensile strength, impact resilience and dimensional accuracy. Even more surprising is the fact that these properties can be obtained without subjecting the sintered products to a heat treatment. It has therefore been discovered that it is possible to obtain sintered products which combine a tensile strength of at least 800 MPa and an impact strength or impact resilience of at least 19 J, using economic sintering equipment, such as a high performance wire mesh conveyor tunnel kiln, operating at a temperature of about 1120 ° C, with sintering times of around 30 minutes. Preferably, the amount of Cr varies between 2.7 and 3.3% by weight, the amount of Mo varies between 0.4 and 0.6% by weight and the amount of Mn varies between 0.09 and 0.3% by weight. The alloy powder steel of the present invention can be produced by submitting steel ingots with the above-defined composition of alloying elements to a water atomization method. Preferably, the powder atomized with water should be prepared in such a way that, before annealing, this powder atomized with water has a weight ratio of 0: C of between 1 and 4, preferably between 1.5 and 3.5, and most preferably between 2 and 3, and a carbon content of between 0.1 and 0.9% by weight. For further processing in accordance with the present invention, this powder atomized with water can be annealed in accordance with the methods described in PCT / SE97 / 01292 (incorporated herein by reference) and which specifically refers to a process that includes following steps: a) preparing an atomized powder with water consisting essentially of iron and optionally of at least one alloying element selected from a group consisting of chromium, manganese, copper, nickel, vanadium, niobium, boron, silicon, molybdenum and tungsten . b) annealing the powder in an atmosphere containing at least H2 and H2O gases; c) measuring the concentration of at least one of the carbon oxides formed during the decarburization process; or d) measuring the oxygen potential essentially simultaneously in at least 2 points located at a predetermined distance from each other, in the longitudinal direction of the furnace; or e) measuring the concentration in accordance with subparagraph c) and also measuring the oxygen potential in at least one point of the furnace; f) adjust the content of gaseous H2O in the decarburizing atmosphere with the help of the measurement. Another process that can be used for the preparation of iron-based powders of low oxygen and carbon content, including low amounts of readily oxidizable alloying elements, is described in Swedish Application No. 9800153-0. This process includes the steps of: - increasing the temperature of the oven, preferably by direct heating or with gas at a temperature between 800 and 1350 ° C; - monitor the increase in the formation of gaseous CO and evacuate the gas coming from the furnace when a significant increase in the formation of CO is observed; - Cool the powder when the increase in CO gas formation decreases.

Subsequently, the annealed powder with low oxygen and carbon content is mixed with graphite powder and optionally with at least one alloying element selected from a group consisting of Cu, P, B, Nb, V, Ni and W, in one quantity that is determined by the final use that will be given to the sintered product. The amount of graphite added commonly varies between 0.15 and 0.65% by weight of the iron-based powder, and a lubricant, such as zinc stearate or H-wax, in an amount of up to 1% by weight of the powder based on iron. This mixture is then compacted at conventional compaction pressures, that is, at pressures between 400 and 800 MPa, and sintered at temperatures between 1100 and 1300 ° C. Preferably and very unexpectedly, however, the products prepared from the powder according to the invention have excellent mechanical properties even if the powders are sintered at low temperatures, that is, temperatures below 1220 ° C, preferably below 1200 ° C or even below 1150 ° C, and comparatively short sintering times, that is, sintering times below 1 hour, such as 45 minutes. Commonly the sintering time is around 30 minutes. The reasons why the respective components in the alloy powder steel and in the sintered body of the invention are limited within certain ranges are the following: the reason why C in the alloy powder steel is not greater than 0.01% is that C is an element that serves to harden the ferrite matrix by forming a solubilization of the solid phase as it penetrates the steel. If the C content exceeds 0.01% by weight, the powder hardens considerably, resulting in a compressibility too poor to obtain a powder that can be used commercially. The amount of C in the sintered product is determined by the amount of graphite powder mixed with the alloy powder steel of the present invention. Commonly, the amount of graphite added to the powders is between 0.15 and 0.65% by weight. With regard to powders with a Cr content of between 3 and 3.5%, the amount of graphite added is a little lower and preferably between 0.15 and 0.5%. The amount of C in the sintered product is essentially the same as the amount of graphite added to the powder. The limited quantities of the following components are shared by both the alloy powder steel and the sintered body. The Mn component improves the strength of the steel by increasing its hardenability or hardenability and by hardening by dissolution. However, if the amount of Mn exceeds 0.3%, the hardness of the ferrite will be increased by hardening by solid phase solubilization, and this, in turn, will result in powders with poor compressibility. If the amount of Mn is less than 0.08 it is impossible to use the waste material that commonly has an Mn content above 0.08%, unless a specific treatment is made for the reduction of Mn during the steelmaking process ( compare with European patent 653 262 page 4, lines 42 to 44). In this way, the preferred amount of Mn according to the present invention is between 0.09 and 0.3%. In combination with C contents below 0.007%, this is the range of Mn that produces the most interesting results. The Cr component is a convenient alloying element in powder steel, since it provides sintered products with improved hardenability without significantly increasing the hardness of the ferrite. A Cr content of 2.5% or higher is preferred to obtain sufficient strength after sintering. A Cr content above 3.5% gives rise to problems with oxide and / or carbide formation. In addition, the hardenability becomes too high, which hinders the practical application of the sintered products, if the Cr content exceeds 3.5% by weight. The importance of selecting this narrow range of between 2.5 and 3.5% Cr to obtain a combination of high tensile strength and resiliency is more fully described in Figure 1 attached hereto. The Mo component serves to improve the strength of the steel by improving its hardenability and also by hardening by dissolution and precipitation. A Mo content below 0.3% has a negligible effect on the properties. Likewise, it is preferable that the amount of Mo does not exceed 0.7% due to the costs of this alloying element.

In general, low quantities, that is below 0.01, of S and P are required, in order to obtain high strength sintered bodies and powders with high compressibility, and the amounts of S and P in the powders used of according to the present invention are below 0.01% by weight. The component O has a great influence on the mechanical strength of the sintered body and preferably the amount of O should be as low as possible. The O forms stable oxides with Cr and this prevents the establishment of an adequate sintering mechanism. The amount of O must therefore not exceed 0.2%. If the amount exceeds 0.25%, large quantities of oxides are generated. Preferably, the sintering of the compacted body is carried out at a temperature below 1220 ° C, most preferably at temperatures below 1200 ° C and most preferably at temperatures below 1150 ° C. As described in the following examples, a good tensile strength is unexpectedly obtained without the need for a subsequent heat treatment, when sintering occurs at temperatures as low as 1120 ° C for periods of 30 minutes only. At elevated temperatures, that is, temperatures above 1220 ° C, undesirably increase the costs of sintering, which makes the powders and the method according to the present invention extremely attractive from an industrial perspective.

A cooling rate below 0.5 ° C / s results in the formation of ferrite, and cooling rates exceeding 2 ° C / s result in the formation of martensite. Depending on, inter alia, the composition of the iron powder and the amount of graphite added, the cooling rates common in tunnel kilns of wire mesh conveyor, that is, between 0.5 and 2 ° C / s, produce structures fully bainitic, which is desirable to obtain a good combination of strength and hardness. In this context, it is worth mentioning that the sintering process according to the present invention is preferably carried out in tunnel furnaces of wire mesh conveyor. The invention is illustrated in greater detail in the following examples.

EXAMPLE 1 A powdered steel with a Cr content of between 2 and 3% by weight, a Mo content of 0.5% by weight and an Mn content of 0.11% by weight was atomized with water, and annealed in accordance with the application of Patent PCT / SE97 / 01292. Graphite (C-UF4) was added in amounts ranging from 0.3 to 0.7% by weight, as well as 0.8% by weight of a lubricant, in this case wax-H. The powders were compacted at 700 MPa and subsequently sintered at a 90% N2 / 10H2 atmosphere for 30 minutes at a temperature of 1120 ° C. The following tables 1, 2 and 3 describe the weight per unit volume of an uncooked tablet (GD), the dimensional change (dl / L), the hardness (Hv10), the tensile strength (TS), the elastic limit or apparent limit of elasticity (YS) and the resilience (Charpy) of the prepared products.

TABLE 1 Powder: 2Cr 0.5Mo 0.11Mn TABLE 2 Powder: 2.5Cr 0.5Mo 0.11Mn TABLE 3 Powder: 3Cr 0.5Mo 0.11 Mn EXAMPLE 2 Too high a content of Mn has a negative impact on the compressibility due to the increase of the hardness of the ferrite by the hardening by solubilization of the solid phase. This is illustrated in Table 2, which describes the compressibility of Fe-3Cr-0.5Mo powder with matrix lubricated at 600 Mpa.

TABLE 4

Claims (12)

NOVELTY OF THE INVENTION CLAIMS

1. - An iron-based powder atomized with water and annealed, which includes% by weight, Cr 2.5 to 3.5, Mo 0.3 to 0.7, Mn 0.09 to 0.3, Cu < 0.10, Ni < 0.15, P < 0.02, N < 0.01, V < 0.10, Yes < 0.10, W < 0.10, O < 0.25, C < 0.01, and the rest is iron and an amount of no more than 0.5% of inevitable impurities.

2. The iron-based powder atomized with water and annealing according to claim 1, further characterized in that it includes, by weight%, Cr 2.7 to 3.3, Mo 0.4 to 0.6, Mn 0.09 to 0.25, O < 0.15, C < 0.007, and the rest is iron and an amount of not more than 0.2% of inevitable impurities.

3. The method for preparing a sintered product with a tensile strength of at least 750 MPa wit a subsequent thermal treatment, which includes the steps of atomizing with water the iron-based powder that includes the Cr, Mo and Mn in the amounts according to any of the preceding claims; anneal the atomized powder with water; adding graphite and optionally at least one alloying element selected from a group consisting of Cu, P, B, Nb, V, Ni and W, in the amount determined by the final use to be given to the sintered product; compacting the annealed powder at a pressure of at least 600 MPa; and subjecting the compacted body to sintering.

4. The method according to claim 3, further characterized in that the reduction is carried out at atmospheric pressure in a reducing atmosphere in the presence of H2 as well as controlled amounts of H20.

5. The method according to claim 3, further characterized in that the reduction is carried out at low pressure in an essentially inert atmosphere and with evacuation of CO.

6. The method according to any of claims 3 to 5, further characterized in that the powder atomized with water before being annealed has a weight ratio O: C of between 1 and 4, preferably between 1.5 and 3.5, and very preferably between 2 and 3, and a carbon content between 0.1 and 0.9% by weight.

7. The method according to any of claims 3 to 6, further characterized in that graphite is added in an amount of between 0.25 to 0.65, preferably 0.3 and 0.5% by weight, to the powder before the compaction step.

8. The method according to one of claims 3 to 7, further characterized in that the powders with a Cr content of 3 to 3.5 have an amount of graphite of between 0.25 to 0.5% by weight.

9. The method according to claim 3, further characterized in that the sintering temperature is at most 1220 ° C, preferably less than 1200 ° C, and most preferably less than 1150 ° C.

10. The method according to claim 3, further characterized in that the sintering times are less than 60 minutes, preferably less than 50 minutes, and most preferably less than 40 minutes.

11. A sintered product prepared according to any of claims 5 to 8, further characterized in that it has a combined carbon content of at least 0.25%, preferably at least 0.3%.

12. The powder according to any of claims 1 and 2, further characterized in that the powder based on annealed iron is prepared according to the method described in PCT / SE97 / 01292.