CS204987B2 - Method of making the steel powder of high current density scattered by water - Google Patents

Abstract

1475421 Making steel powder USS ENGS & CONSULTANTS Inc 6 Aug 1974 [16 Aug 1973] 34650/74 Heading B3A A method of making steel powders having high apparent densities from water atomized steel particles comprises annealing the particles at 1400-2100‹ F. to soften them and reduce their oxygen content to below 0À2 wt. per cent, the annealing causing the particles to sinter together, and grinding the annealed sintered particles in a disc mill to produce powder with an apparent density in excess of 3À2 gm./c.c. and a particle size generally finer than 80 mesh. The atomized particles may contain up to 0À8% C, but preferably less than 0À15% C, and the carbon content is lowered by annealing in a decarburizing atmosphere. The initial oxygen content of the particles after atomizing is about 1% and they pack to a high apparent density, i.e. in excess of 3À2 gm./c.c. After annealing, which is carried out in a reducing atmosphere, e.g. hydrogen or dissociated ammonia, the apparent density is 2À8-3À2 gm./c.c. and the particles are in the form of a sintered cake. The cake is broken into small pieces which are fed to the centre of a disc mill, the resulting particle size being determined by the mill gap and the speed of rotation of the discs.

Description

The present invention relates to a method of producing a water-scattered high-density steel powder, wherein water-scattered steel particles are produced with a predetermined size distribution, wherein at least 80% of said particles are finer than for 80 mesh meshes, and said distribution has a particle size P value of 2.0 to 4.0, the particles are softened by heat treatment at 760 ° C to 1160 ° C and their oxygen content is reduced to less than 0.2% wherein the particles are sintered by heat treatment.

Various methods for producing metal powders are currently used. The metal powders can be produced by the electrolytic process or by direct reduction of metal oxides or reduction of metal halides, or by cleavage with high pressure liquids, i.e., water and inert gases. Reduction of metal oxides and water dispersion are clearly more economical for producing large amounts of steel powders. The other of the two processes, i.e. the production of steel powders by water dispersion, has the resulting product with a lower metal content. Thus, the water-dispersed powders exhibit better properties than the liquid, i.e., better efficiency in feeding into the press process, and therefore allow higher production parameters to be achieved in the production of powder metallurgy processes. An example of a water dispersion method is given in U.S. Pat. No. 3,325,277. Although this method has many. However, there is a certain limit to the mechanical properties of the powders that can be produced in this way. The density of commercially used water-dispersed steel powders normally ranges from 2.8 g. Cm ^-3 to 3.2 g. cm ' ⁵ . The density is given by measuring the weight of the powder in a calibrated vessel. Since the density can be influenced by the packaging method, this measurement has generally been standardized and is performed as a measurement of powder flow through a 2.54 mm diameter and 3.165 mm diameter hole located 25.4 mm above the top surface of the container having a content of 25 mm. cm ³ .

A process for producing high-density cast iron powder is disclosed in U.S. Patent 3,597,188. However, this process is limited to the production of coarse powders that are brittle due to the carbon content. While most powders produced by water dispersion have a fineness of about 80 mesh, most of which are finer than 200 mesh, this process is uneconomical because more than half of the powder initially produced has the following note. way. The total weight in the fractions is first removed from those particles which have been retained on sieves of 100, 140, 200, 230 and 325 and then the remainder of the bowl. Then, the fractions thus determined are summed and divided by 100. In this way and using this definition, a larger particle size characteristic P corresponds to a coarser particle size distribution and a low particle size characteristic P a fine particle size distribution.

For example, the particle size characteristics P of the powder will be calculated as follows.

The number of mesh meshes retained by wt.

Total retained% wt.

100 ALIGN! 2.4 2.4 140 5.3 7.7 200 16.1 23.8 230 4.9 28.7 '325 12.9 41.6 bowl 58.4 100.0

100.0

Therefore, the value of the particle size characteristics P for this powder will be 204.2 / 100 = 2.04. Water-comminuted particles having a particle size characteristic P of about 2.0 to 4.0 can be effectively used for this process.

Once the value of the particle size characteristics P is known and the powder has been heat treated, a comminution cycle can be introduced to provide properties for special requirements.

A disk friction mill is used to achieve the desired degree of comminution. The heat treatment causes the particles to sinter together to form flakes. If necessary, they first break into pieces small enough, usually less than about 25 millimeters, to be inserted into a disk mill. In this mill, the comminution is carried out between the discs, which rotate in either a vertical or a horizontal plane. The cartridge enters near the center of the disc and proceeds by centrifugal force to the peripheral portion of the comminution plate and is then distributed. While spike-toothed plates are used in some disc mills, it should be noted that these plates cannot be used in the process of the present invention, where use is limited to conventional friction-disintegrating plates.

The term "grinding gap" as used herein means the distance between the grinding plates. The disc mill is particularly adapted for the purposes of the invention because it has been found that this type of mill is capable of exploiting it from controlled and predetermined comminution, which is essentially a grinding function. gaps and linear velocities on the mean radius r _{m of the} crushing plates. In a disk mill, the grinding plate bearings form a ring, i.e. two concentric circles; for these, the distance from the center to the crushing plate, i.e. from the center to the inner circle, is equal to ru. The distance from the center to the circumferential portion of the crushing plate, i.e. from the center to the outer circle, is equal to гг. Then the mean radius r _{m is} given by - (π + Γ2) / 2.

204.2

Since the linear velocity v is equal to the angular velocity ω multiplied by the radius, the linear velocity of the point on the mean radius rm can be easily determined from the frequency-rotation of the crushing plate. Thus, for example if they are grinding plates -se -středním- radius rm = -304.8 mm speed 3000 -min ^'1, the linear velocity equal to v = w.rm =' 3000th .6,28.304,8.10-3 = 5742,33 -m.min “ ¹ .

Using statistical regression and interpretation analysis, the effect of the variables on the normal density of the final powder product appears to be expressed by the following equation: Normal density (g.cm- ³ ) = 2.1332 + + 0.30.P -2,1516l.'W- ² + 45591.10 LNG-lvr.P. + + 2,23C ^ - 10O- ⁴ · v2-9,3543 / 11 ^ -3v.l ^ r ^ G, where P is the characteristic particle size of the water-ground powder before heat treatment, v is the linear velocity of the point on the mean radius the grinding plates (mm.min ^_1) and Bc is the natural logarithm of the size of the grinding gap -udané in (mm).

Using the above equation, the comminution process can be performed so as to guarantee the properties of the powder as a final product for special requirements. In order to better understand the application of this equation, the graphs according to FIG. 1 and 2 for a laboratory mill size with a disk having a diameter of 330 mm and a center radius _rn = 134.6 mm.

For easier interpretation, i.e., to avoid cumbersome operations with large numbers, the linear velocity was converted to the rotational speed (min- ¹ ) of the disk-mill. The graphs of FIGS. 1 and 2 can be used - only - for the results of a disk mill with a mean radius of? 134.6 mm.

In practice, the commonly used disc mills larger medium poloměru- R _IN. The curves in Figures 1 and 2 should then be shifted to lower rotational frequencies as the mill size increases, while the linear velocity - at the specified rotational speed would increase accordingly. Normally, the operation of these mills is such that - the frequency of rotation is between 200 and 5000- min- ¹ and a grinding gap of 0.254 to 2.54 millimeter.

must-remove. Equally important, the process is not suitable for the production of steel powders, i.e., those having a carbon content of less than 1.7% by weight.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a process for the production of water-dispersed steel powders at a density equal to or greater than

3.2 g. Cm- ³ , with the bulk of the dispersed powders being utilized.

It is a further object of the present invention to provide a process for producing steel powders for a molding step which, in combination, has a high conventional density and high green strength sufficient to allow normal powder processing prior to sintering.

The invention solves these tasks -The said heat -Processing -se sintered particles are ground in a disc -mlýně whose rotation frequency is between 200 and 5000 rpm and ⁴ whose grinding gap has a size of from 0.254 mm to 2.54 mim on - on powder - a normal density of more than 3,2 g. cm ~ ³ , with a particle size smaller than 80 mesh of the sieve.

According to a preferred embodiment of the invention, the linear velocity v (m.s-i) on the mean radius of the grinding mill and the grinding gap G [mm] are correlated with the particle size characteristic value P according to the following equation:

2.1332 + 0.30. 2,1-Ρ! ^ ^ (5.10 ~ 2. Lng + 4.5591. IO ^"3. V. + P. 10 ^-4. In ² - -9.3543 IO '3. V. lnG> 3.2

According to a further preferred embodiment of the invention, the amount of water-dispersed particles finer than that of 80 mesh is at least 95% by weight, with a substantial portion finer than 200 mesh.

By these measures according to the invention it is achieved that the steel powder production process according to the invention yields steel powders having a combination of high conventional density and strength, allowing normal pre-sintering processes.

The invention will now be described with reference to the accompanying drawings, in which FIG. 1 is a graphical representation showing the effect of distributing normal density (g.cm ^-3 ) according to particle size P (1) and linear velocity v on the center radius of the disc. mill, given by the rotation speed (min ”1), for the size of the grinding gap - 1.58, - fig. -2 is a graphical representation for a grinding gap size of -0.39 mm.

The process according to the invention, applicable to water-dispersed steel powders, applies in virtually any source. The water-dispersed steel powders usually contain impurities, primarily in the form of oxides, which must be removed before the powder is sold and manufactured by powder-metallurgical processes. In order to manufacture powders of alpha-Eli highest possible compressed ^'and nose ^{of children is desirable,} and ^would onečné ^p Rašková product had a carbon content less than 0.10 weight percent, and most preferably below 0.01% wt.

In general, however, it is impractical to produce a steel melt with such a low carbon content as a starting product. Therefore, the steel melt can contain up to 0.8% by weight of carbon, but less than 0.15% by weight of carbon is most preferred, wherein the carbon content of the dispersed powder is additionally reduced by heat treatment in a reducing environment. The dispersion by the water jet at high pressure results in the possibility of rapid reduction of the dispersed metal particles at the beginning of the dispersion process. If low carbon steel is used in the scattering process, i.e. to avoid the need for subsequent decarburization, it is still necessary to heat-treat the powder to both soften and reduce the carbon content therein, and % to less than 0.2 wt.

The initial oxygen content of the water-dispersed particles is usually in the range of at most 0.2 wt%, and usually about 0.1 wt%.

As a result of this high oxygen content on the surface of the particles and its distribution, the scattered particles combine to a considerable normal density, i.e. mainly near 3.2 grams. cm ⁴

After heat treatment and a reduction in the oxygen content, the usual density values are in the range of - 2.8 g / cm @ 2 to 3.2 g / cm @ ³ . The heat treatment is carried out at temperatures from 760 ° C to 1150 ° C in a reducing environment, for example hydrogen or dissociated ammonia, for a time sufficient to cause the desired softening and reduction of impurities. This heat treatment process not only cleans the steel powder, but causes the particles to stick together in the form of sintered flakes, which makes it necessary to split, crush and bring back to the powdered state. In the process of U.S. Pat. No. 3,352-277, this necessary crushing is carried out in a hammer mill; impact crushing is used to bring the particles back to their original size. The invention removes the crushing operation itself by using a disc mill according to the invention; the invention uses a cleavage mechanism to spread to dust. It has been found that by regulation - this crushing operation - the final normal density can be adapted to the particular requirements which depend on the size distribution of the original dispersed particles.

The particle size distribution of the powdered particles may be determined by conventional separation analysis. This partition analysis is used to determine the characteristics of the P powder sizes. It has been found that the varying coarseness of the dispersed particles cannot be the basis for achieving the desired effect of the invention. In order to achieve the desired degree of crushing, it is necessary that at least 80% and preferably more than 95% of the particles be finer than 80 mesh. While many different methods are useful for measuring the value of particle size characteristics P, this invention has been determined for the purposes of the present invention.

Powder 100

140 200 230

325 A bowl P of the finely processed sintered particles is ground in a disk mill, the frequency of which the graphs used in Figs. 1 and 2 will be described for powders in the following example of a distribution analysis:

A 10.4 18.0 26.0 6.2

B 4.4 9.5 21.0 6.0

When ground powder A is used and the mill mill is operated with a grinding gap of 1.58 mm, it can be determined, for example, that the normal densities are 3.2 g.cm ³ and 3.45 g.cm ^-3 can be achieved, if used · rotational speed ranging from 2500 to 3650 min ^-1.

The effect of reducing the grinding gap appears when compared to FIG. 2. If the same powder A is used, similar densities can be achieved at a substantially lower wheel speed. Thus, at a rotational speed of 1600 rpm ^{, the} product will have a density of 3.2 g.cm ^-3 , while at a density of 3.45 g.cm ^{-3, a} rotational frequency of 2775 rpm is sufficient. Powder B, which is finer, cannot provide high conventional densities in a small diameter disc mill. However, if the grinding gap is reduced to 0.39 mm, by sweat according to FIG. 2, equivalent conventional

15.3 24.1 3.29

17.0 4 ^ ZL63 2.52 tot at 3225 mn-1 or 4050 rpm.

From the examples above or even from the equation itself, it can be deduced that the normal density increases when the particle size characteristics P increases, when the rotational speed of the disc increases at the mill, and finally when the size of the grinding gap decreases.

It has also been found that in the temperature range from 760 ° C to 1150 ° C, the normal density can be increased by lowering the temperature for heat treatment. Then, however, the processing times must be longer at lower temperatures. A practical compromise between these requirements, i.e. the achievement of high density and relatively short heat treatment time, is in the temperature range between 926 ° C and 1038 ° C.

Claims (3)

OBJECT OF INVENTION A method of producing a water-scattering high-density steel powder dispersed in water, which produces water-scattered steel particles having a predetermined size distribution, wherein at least 80% of said particles are finer than for 80 mesh meshes, and said distribution has a value of: according to the particle size characteristic P in the range of 2.0 to 4.0, the particles are softened by heat treatment at a temperature of 760 ° C to 1150 ° C and their oxygen content is reduced to a value of less than 0.2% by weight; heat-treating the particles together by sintering, characterized in that the heat-treated sintered particles are ground in a disk mill whose rotational speed is between 200 and 5000 rpm and whose grinding gap has a size of · 0.254 mm to 2.54 mm, for a powder of conventional density greater than 3.2 g.cm-3, with a particle size smaller than 80 mesh. 2. Method according to claim 1, characterized in that the linear velocity v (ms ^-1 j at the mean radius of the grinding wheel and the grinding gap G '(mm) are in relation to the value of the particle size characteristic P in accordance with this equation. : 2.1332 + 2.1516-0,30.P. .l0- LNG ² + 4; 5591.10-3.vP 2,232.l0 ^-4 .in + ² - -9,3543.10-3. v.lnG> 3 , 2. 3. The method of claim 2, wherein the amount of water dispersed particles finer than 80 mesh is at least 95% by weight, with a substantial portion finer than 200 mesh.