RU2383404C1 - Hard alloy die for production of strip bar - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: die consists of drawing channel with deforming zones created with big and small facets. Angle of tilt of the smaller facet to lengthwise axis of the channel exceeds angle of tilt of the bigger facet to lengthwise axis of channel so, that length of a squeezing section of the deforming zone of the bigger facet exceeds length of the squeezing section of the deforming zone of the smaller facet.

EFFECT: increased durability of die, raised stability of process of drawing strip bars out of hard deformed alloys, improved quality of their surface.

1 ex

Description

The invention relates to the field of metallurgy, in particular to the production by drawing of strip profiles of nichrome and similar hard-deformed alloys and steels, namely, to the shape of the drawing channel of carbide dies used in the specified production.

It is known from the prior art [1-11] that practically all bibliographic sources contain information on taper angles (tilt angles) 2α of the deformation zone of the drag channel, where α is the angle of inclination of the generatrix of the deformation zone to the longitudinal axis of the drawing channel. However, despite the significance of this parameter, widely known from the theory and practice of drawing, information sources contain significantly contradictory information regarding the specific values of this angle.

In particular, in [1, p. 170], an angle 2α = 12-16 degrees was recommended for drawing wire from low carbon steel; in [1, Table 24] the values 2α = 8-14 degrees are given; in [2, p.182], the authors adopted an angle α = 8 degrees. A much wider range of 2α for drawing low-carbon steel is given in [3, Table 21] - from 6 to 20 degrees. A wide range of angles was also proposed in [4]: for medium-carbon steels 2α = 6–18 degrees [4, Table 3], for steel EI903 from 8 to 14 degrees [4, p. 40], for wire made of nickel alloy diam. ≤2 mm 8-10 degrees, for wire dia.> 2 mm 10 degrees

[4, table 4]. Almost the same values of 2α = 10–12 degrees are recommended in [5, Table 93]. For drawing of bars to dia. up to 40 mm according to [7, p.305] dies with α = 6–9 degrees are used, and in accordance with [6, p.76–77], for those rods, the angle 2α for various metals and alloys should be within the following limits, hail: aluminum, zinc 24-26; copper, silver 16-20; bronze 14-18; nickel and its alloys 12-16; titanium, tungsten, molybdenum 8-10. Also of interest are the recommendations on α proposed in [10, p.218] and relating to the following metals and alloys, grad: aluminum, silver 16-18; copper 12-16; brass, bronze 9-12; steel 5-11. A unique approach to the selection of α depending on the material of the dies in the production of calibrated steel was proposed in [11, p. 97]: α = 12–20 degrees recommended for steel dies, 6–12 degrees for carbide ones. An extremely wide range of angles 2α = 10-32 degrees when drawing steel bars was studied in [8].

The most complete and substantiated by laboratory experiments and industrial tests data on the angles of the deformation zone of the drawing channel are given in the fundamental monograph [9], however, the authors were not able to avoid some inconsistency of the given information in this source as well. In particular, the following values of α, hail, are recommended: 6-12 (p. 110, Table 4); 4 ° 10'-21 ° 30 '(p.118, table 5); 3-15 (ibid.); 8 ° 40'-13 ° 30 '(p. 121, Table 7); 4-13 (p. 144, table 14); 6-18 (p. 436, adj. 3).

Summing up the analysis of the prior art in the considered direction, we can state the following:

- such wide ranges of angle values are generated by both the features of drawing products of various shapes and sizes of cross sections from various alloys, and the specifics of a particular production, as well as the technical and technological capabilities of tool sections of drawing shops;

- the technical and patent bibliography presents in sufficient detail data (albeit very contradictory) on the values of the angles of the deformation zone of the dies, designed to produce wire, round rods and some simple profiles, but the situation with respect to strip profiles, in particular, a rectangular section, is not considered;

- when drawing strips, including rectangular billets obtained by hot rolling in calibers are used, however, when designing channel profiles, it should be borne in mind [9, p.275] that at equal angles of inclination of all the contact surface components to the channel axis, a difference in contact lengths is inevitable surfaces in different parts of the deformation zone. Therefore, the boundary of the entry of the workpiece into the channel cannot fit on a plane perpendicular to the axis of the channel, and therefore, the deformation of the metal in different sections of the cross section does not start simultaneously, as a result of which production, after drawing, sometimes has noticeable defects [9, p.275].

The features of using the tool to obtain rectangular and close to them (in particular, trapezoidal) profiles, as well as the specifics of the metal flow briefly described above during its deformation in the channel [9], become much more complicated if we switch from consideration to processing electrical products from high ductility alloys and low strength (rectangular copper busbars and trapezoidal collector strips of low-alloyed copper alloys) to the situation arising from the drawing of strip profiles from rudnodeformiruemyh alloys such as nichrome. Well-known processing methods and their corresponding design solutions for drawing tools, implemented in the production of high-quality products from copper and copper alloys, are of little use for less ductile and significantly more durable alloys such as nichromes and a number of high alloy steels. It is appropriate to pay attention not only to a sharp increase in the force parameters (stress and drawing force, metal pressure on the fiber), but also on another feature of these steels and nickel alloys, which significantly complicates their deformation, namely, the high adhesive ability of the metal to the fiber material - even with preliminary coating of the surface of the workpieces with lubricant layers and the use of an effective lubricant in the form of sodium soap powder with additives. These circumstances, accompanied by uneven compressions along the large and small faces of the profile, cause intense wear of the deformation and gauge zones, which reduces the tool life, worsens the surface quality of the stretched strips and increases their breakage in the process of drawing.

The problem of obtaining nichrome strips meeting the requirements of GOST [12] by drawing appeared for the following reason. Due to the sharp decrease in the last 15-17 years, the volumes of metal consignments ordered by ferrous metallurgy plants by Russian consumers of these products, serial production of nichrome strips, in which hot and cold rolling of ingots of significant weight (up to 500 kg) were used to produce cold rolled rolls and their subsequent cutting into narrow strips according to orders, was transformed into unprofitable. Given this circumstance, in the production of the applicant, after conducting appropriate research, a highly efficient technological regulation was developed for small-scale production of strips according to the following briefly described technological scheme:

- smelting of the alloy in an induction channel furnace using raw materials, fluxes, deoxidizers and modifiers established by a charge card;

- production of conical shaped ingots weighing 55-60 kg by filling casting;

- heating of the ingots in the chamber furnace and their hot rolling on a small-grade wire mill to strip billets with the winding of the latter in riots;

- etching of hot-rolled billets with appropriate rinses, applying a lubricant coating, sharpening the front end;

- drawing blanks to strips of finished sizes in several passes on a single drum drawing mill through carbide dies with intermediate annealing after each drawing pass, sodium soap grease-powder with additives.

The aforementioned harsh conditions for the drawing of strips of difficult-to-deform alloys are confirmed by the accumulated negative manufacturing experience of the applicant, who had previously used carbide dies with a channel in industrial batches of nichrome strips, in which the lengths of the compression sections of the deformation zones along the large and small sides of the channel can unpredictably vary depending on many factors, including the distribution of absolute reductions in thickness and width of a rectangular profile, with equal, as a rule, the angles of inclination of these faces to the axis of the drawing channel.

As the closest analogue of the claimed technical solution, a die was chosen to obtain rectangular and wedge-shaped (trapezoidal) profiles with a channel, the shape of which is given in [9, Fig. 126, 127, 145]. Relative to the dies shown in Figs. 126, 127, 145 in [9], explicit information does not contain information on the absolute values and on the ratio of the tilt angles of the large and small faces of the drawing channel to its longitudinal axis, however, from the consideration of Figs. 126, 127 , 145, and also on the basis of the process of obtaining the calculation formulas given on pp. 229-232, the conclusion about the equality of these angles follows.

Otherwise, the authors of [9], taking into account the thoroughness and depth of the study of all material on the subject of the monograph, had to necessarily and purposefully draw the attention of specialists to this issue, which is important for theory and practice of drawing. Thus, based on the analysis of the information contained in [9], we can conclude that the large and small edges of the channel according to the known technical solution have equal angles of inclination to the axis of the channel. This ratio of angles leads to the following disadvantages.

1. In the current production it is extremely difficult, and in most cases it is practically impossible to carry out drawing with equal absolute reductions along the large and small faces of the drawn profile for at least two reasons:

- firstly, due to the considerable size of the tolerances on the thickness and width of the hot-rolled billet, which in the most favorable case are ± 0.5 mm;

- secondly, due to the difficulty of controlling many drawing conditions in real production and, accordingly, the difficulty of controlling them (misalignment of the specified workpiece, strip and channel; vibration of the drawing device of the drawing mill; insufficient lubrication into the channel; metal sticking to the channel surface followed by hard alloy coating in the deformation and calibrating zones of the channel, etc.).

2. If the absolute compression along the large face of the profile is higher than along the small face, then the length of the compression section of the deformation zone of the large face is greater than the corresponding length of the compression section along the small face, therefore, deformation of the workpiece on large faces will begin earlier than on small faces. This is a positive point in the situation under consideration, since it does not lead to negative consequences from the position of the metal flow in the deformation zone, subject to other conditions of drawing.

The situation deteriorates sharply if, for some reason (in particular, as set forth in paragraph 1 above), the absolute compression along small faces exceeds the absolute compression along large faces. In this case, the length of the compression section of the deformation zone of the small face becomes larger than the corresponding length of the compression section along the large face, and the metal deformation begins on small faces earlier than on large faces, which, taking into account the reasons stated below, gives rise to the following disadvantages:

- there arises an extremely unfavorable phenomenon of transverse bending of a wide strip due to the almost inevitable loss of stability with a significant ratio of width b to thickness h, which, for example, for nichrome strips manufactured by the applicant, ranges from 6: 1 to 15: 1. This loss of stability can lead to the formation of longitudinal folds in the stretched strip, that is, to an irreparable defect;

(где Δh - абсолютное обжатие, R - радиус валка); h_c - средняя высота ОД, равная (h₀+h₁)/2 (где h₀ и h_I - соответственно начальная и конечная высота полосы). Применительно к процессу волочения С является длиной обжимающего участка деформационной зоны, а функцию h_c выполняет b_с - средняя ширина протягиваемой полосы.- the flow of metal in the deformation zone (hereinafter referred to as OD) with the beginning of early (premature) deformation along small faces is such that the deformable volumes of the metal are very little displaced in the longitudinal direction, that is, along the drawing axis, and preferably are displaced in the transverse direction, similar to the well-known The elementary theory of rolling an intense metal flow in width under conditions of very high OD, when the shape factor OD € l / h _c << 1, where l is the OD length calculated by the well-known formula

(where Δh is the absolute compression, R is the radius of the roll); h _c is the average OD height equal to (h ₀ + h ₁ ) / 2 (where h ₀ and h _I are the initial and final height of the strip, respectively). With reference to the drawing process, C is the length of the crimping portion of the deformation zone, and h _c performs the function b _s - the average width of the stretched strip.

The situation under consideration, when the radius of curvature is limited in the standard at the corners of the deformation and gauge zones of the channel, that is, when the displaced metal is difficult to flow into these angles, in the worst case leads to the appearance of "excessive" volumes of metal in the form of waves on large strip faces and is almost inevitable its breakage or squeezing out the lubricant from the OD in the direction opposite to the direction of drawing, with the formation on the strip of shiny areas, indicating the absence of lubricant on their surface during drawing. It is clear that these negative factors dramatically increase the wear of the die channel, worsen the surface quality of the strip, and lead to frequent breaks during drawing.

The task of the invention is to increase the stability of the process of drawing strip profiles from difficult to deform alloys, improve the quality of their surface and increase the resistance of the dies.

The problem is solved in that in the known carbide fiber containing a drawing channel, including a deformation zone formed by large and small faces of the drawing channel, made with almost equal angles to the longitudinal axis of the drawing channel in such a way that the compression sections of the deformation zone, respectively, large and small faces have almost equal length, according to the proposed technical solution, the angle of inclination of the small face to the longitudinal axis of the drawing channel exceeds the angle of inclination on the large faces to the longitudinal axis of the drawing channel so that the length of the deformation zone confining portion large faces confining portion greater than the length of the deformation zone of small facets.

From the formula of the claimed invention it follows that according to the proposed technical solution, the angle of inclination of the small face to the axis of the channel should be greater than the angle of inclination of the large face, which unambiguously leads to a situation where the length of the compression section of the deformation zone of the large face exceeds the length of the compression section of the deformation zone of the small face , deformation of the workpiece metal along the large face begins earlier than along the small face, and thus all the negative effects set forth in the section are guaranteed to disappear e "Criticism of the closest analogue." This fact was confirmed in the applicant’s production, in which the industrial production of 14 positions of cold-drawn strips of nichrome grade X20H80-N was mastered.

It was noted above that the difficulties of constructing a drawing tool for processing difficultly deformed alloys are caused by two circumstances:

- high level of strength characteristics and reduced - plastic; for example, the tensile strength of cured nichrome reaches 1200 MPa, and for stainless steel X18H9T is 1200-1300 MPa;

- a pronounced tendency to adhesion of nichrome and a number of high alloy steels and non-ferrous metal alloys to the surface of carbide fibers, and the sticking of the drawn metal to the hard alloy stably takes place, despite the use of lubricating coatings and the use of dry powder lubricants with appropriate effective additives.

The situation is aggravated by the high normal contact pressure of the metal on the die, contributing to the extrusion of lubricant from the metal contact zone with the channel surface. This is confirmed by the calculation below.

The maximum absolute compression over large faces Δh _max when drawing strips in the production of the applicant is 1.0 mm Therefore, when the angle α _min = 6 ° and tan α _min = 0.1051 is accepted for known dies, for example, the maximum OD length calculated by the formula l ^max _od = (Δh _max / 2) / tg α _min is l ^max _od = (1.0 / 2) / 0.1051 = 4.76 mm and is essentially the length of the compression portion of the deformation zone of the large face. According to [9, p.143, formula VI-8a] the average value of the normal voltage on the contact surface is calculated by the formula

,

,

where σ _lup - longitudinal stress in the elastic zone, MPa; E is the modulus of normal elasticity, MPa; ν is the Poisson's ratio; α is the angle of inclination of the deformation zone to the channel axis, deg; µ is the coefficient of friction.

When choosing the parameter values in the equation for calculating p _n, due to the absence in the bibliography of nichrome data, guided by the applicant from titanium VT1 [9, Table 14] as closest to the mechanical properties nichrome. These parameters are: σ _lup = 64 MPa; E = 1.1 · 10 ⁵ MPa; ν = 0.22; µ = 0.07 [9, p. 439-440, adj. 6]; the angle α is taken equal to 6 degrees. As a result, the calculated value of p _n = 1650 MPa was obtained, which correlates well with that given in [9, Table 14].

Based on an additional calculation of the average contact pressure p _n in OD, performed according to [13, p. 298] using the data from [13, p. 299, table 22], a value of p _n = 1510 MPa is obtained, which is in satisfactory agreement with the result calculation according to [9, table 14].

The surface area of the crimping section of the large face in the form of a trapezoid when drawing the workpiece, for example, with a cross section of 3.5 × 40.5 mm into a strip of 3 × 40 mm, will be F = [(b _o + b ₁ ) / 2] · l _od , where b _o and b ₁ - the width of the workpiece and the elongated strip, respectively; l _od - the length of the OD, component according to the above calculation

l ^max _od = 4.76 mm. Then F = [(40.5 + 40) / 2] · 4.76 = 191.6 · 10 ^-6 m ² and the radial normal force P = p _n F = 1650 · 191.6 = 316 kN.

That such high values of the specific p and _n P the full pressure on the metal wall of the channel of the die, and the tendency of nickel alloys to adhere to the first lead of the extrusion lubricant OD, and then - to intensive wear surface channels. In particular, the following was observed in the applicant’s production: in the beginning, the drawn metal sticks to the large and small edges of the channel, and then the hard alloy is chipped in the deformation and gauge zones (Fig. 3). An uninterrupted supply of lubricant to the OD and its presence between the drawn metal and the channel surface, that is, the reliable creation of a stable boundary friction regime, is prevented by a significant level of metal pressure on the die - as a consequence of the physical nature of the alloy being processed. In addition, it should be noted that focusing on the creation of a hydrodynamic friction regime is clearly inappropriate here because of the low drawing speeds of strip profiles (70-160 m / min).

Since the applicant is not able to change the physical nature of hard-deformed alloys, and a certain level of their mechanical characteristics, as well as a tendency to adhesion should be taken for granted, when solving the problem posed in the claimed invention, the applicant directed his efforts to find such a technical solution, which to a large extent would compensate for the impact of the mentioned negative factors. The practice of applying the technical solution found in the applicant’s working conditions confirmed its acceptability and effectiveness, namely: industrial batches of 14 positions of strips made of nichrome grade X20H80-N are produced that fully meet the technical requirements of the standard [12] for all qualitative and quantitative indicators. In the drawing cycle of the production of strips, carbide dies with a channel exactly matching the claims of the claimed invention are used.

The task of increasing the stability of the drawing process by eliminating breakage is solved as follows. As you know, a break in the front stretched end of the strip is a consequence of exceeding its safety factor k = σ _b1 / p, where σ _b1 is the tensile strength of the strip material in a state hardened depending on the degree of deformation; p is the drawing voltage. This coefficient for the implementation of a stable drawing process should be at least 2-2.5 [2, 4, 7, 9, etc.], since in the conditions of the current production there are always a considerable number of negative factors: crescent and waviness of the hot-rolled billet, annealing mode deviation from regulated, insufficiently clean removal of scale by etching after intermediate oxidative annealing, insufficient supply or inadequate lubrication quality, vibration of the drawing device of the drawing machine and other dynamic effects, misalignment of the channel and the direction of application of the drag force, etc., and when using the known die, another negative phenomenon described above additionally appears that increases the probability of breakage, which is confirmed by the practice of manufacturing industrial batches of strips in the applicant’s production.

It should be noted that it is particularly difficult to extract from the die the back non-stretched end of the workpiece that is stuck in the channel during a break. Due to the high strength properties of cured nichrome and its high tendency to adhesion, this stuck end is so tightly clamped in the channel that manual removal is not feasible, and you have to use a bench tool - a chisel and a fairly massive hammer. In case of accidental impact on a die from an expensive, but brittle hard alloy, it is destroyed. All this additionally causes significant material losses and increases the time required to remove the stuck end, leading to a decrease in labor productivity in the drawing process.

As a result of using a die with the proposed parameters of the drawing channel, due to a more favorable distribution of the metal flow in the plastic deformation zone, the breakage is practically eliminated and thereby its negative consequences, in particular, a decrease in productivity, are eliminated.

Improving the surface quality of the strips when using the claimed technical solution was achieved by creating a boundary friction regime on the entire strip surface, including by eliminating the "excessive" volumes of metal concentrated in the deformation zone and preventing the lubrication (sodium soap powder with additives) from entering the OD. In addition, shiny areas completely disappear from the strip, indicating the absence of lubrication on their surface during the drawing process, which previously contributed to intensive wear of the channel. By drawing through a draw with the claimed channel parameters, strips with a smooth matte surface are obtained, which confirms the fact of a stable supply of lubricant to the entire surface of the strip during its drawing. The above information indicates a solution to the problem of the claimed invention in terms of improving the surface quality of cold-drawn strips of difficultly deformed alloys.

As for the solution of the third part of the task of the claimed invention, namely, to increase the resistance of the dies, it is necessary to note the following. The maximum resistance of the die, made according to the proposed technical solution, reaches 3500-3800 kg of stretched strips, in contrast to the previously used tool, made with the channel geometry according to the known technical solution, when the die is damaged due to intensive wear in the best case after drawing 500 kg of metal, and in the worst case, after drawing two or three riots of the workpiece with a mass of one riot ~ 50 kg. In this case, frequent breaks of the front end of the strip occurred with all the negative consequences following from this.

Thus, the above information confirms the positive solution to the problem of the claimed invention: in terms of improving the stability of the process of drawing strip profiles from difficult to deform alloys by eliminating breakage; in the direction of improving the quality of their surface; to increase the operational stability of the die.

The following is an example of a specific implementation of the claimed invention related to the production of a strip of nichrome with dimensions of the finished section 3 × 40 mm, the manufacture of which used carbide dies with a channel made in accordance with the proposed technical solution. In the example, the first drawing pass is examined, carried out along the route, mm: 6 × 42 → 5 × 41.2 and chosen because the very unfavorable conditions for observing the equality of absolute reductions in thickness and width of the strip arise in the first pass, which is generated the maximum tolerances for the hot-rolled billet equal to ± 0.5 mm, while for intermediate billets, and even more so for the finished strip, they are significantly narrowed. Below are the initial data for the calculation.

1. Calculation formulas for determining the lengths of the compression sections of the deformation zones of the large and small faces

l _h = (Δh _max / 2) / tg αh; l _b = (Δb _max / 2) / tg α _b ,

where l _h and l _b are the lengths of the compression sections of the deformation zones along the large and small faces, respectively, mm; Δh _max and Δb _max are the maximum absolute reductions in thickness and width, respectively, taking into account plus tolerances, mm; α _h and α _b are the angles of inclination of large and small faces to the axis of the drawing channel.

2. According to the drawing route Δh _max = 1.0 mm; Δb _max = 0.8 mm.

3. The following angles are accepted:

l _h = 4 ° (tg4 ° = 0.07); l _b = 12 ° (tg12 ° = 0.2126).

In the most unfavorable combination of parameters (the thickness of the hot-rolled billet made in the minus tolerance is 5.5 mm and its width made with the plus tolerance is 42.5 mm), the actual absolute reduction in thickness Δh will be 5.5-5.0 = 0.5 mm; the actual absolute reduction in width Δb will be 42.5-41.2 = 1.3 mm. Actual length l _h = (0.5 / 2) / 0.07 = 3.57 mm; actual length l _b = (1.3 / 2) / 0.2126 = 3.06 mm. Thus, it was obtained that l _h > l _b , therefore, the distinguishing feature of the claims of the claimed invention is met, that is, the length of the compression section of the large-area deformation zone exceeds the length of the compression section of the small-area deformation zone. In the second and subsequent passes, the tolerances are sharply narrowed (h ± 0.4 mm; b-0.6 mm), as well as in the finishing pass (h ± 0.2 mm; b-0.3 mm), therefore, violation of the condition l _h > l _b in these passages is excluded, which is obvious without additional calculations.

The drawing channel of carbide dies in the applicant’s production is obtained by means of an EDM cutting process using a precision machine model AGIECUT CLASSIC 2S (Switzerland) using a wire made of brass grade L63 with a diameter of 0.25 mm as a consumable electrode.

The grade of carbide is usually VK8. The machine is equipped with the following devices and systems:

- highly adapted software system;

- feedback of the processing center with the control computer, carried out using optical devices;

- a system for maintaining a stable temperature regime of the working medium — distilled deionized water — in order to minimize thermal distortions and stabilize the parameters of the electric arc;

- air conditioning system;

- high-precision devices providing step discreteness in the process of EDM cutting, equal to 0.1 microns.

In this regard, obtaining the angles of inclination of the large and small faces of the deformation zone to the channel axis, required in accordance with the claimed technical solution, is quite reliable, that is, with the necessary accuracy.

INFORMATION SOURCES

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Claims (1)

Tungsten carbide for producing strip profiles from hard to deform alloys of the nichrome type, containing a drawing channel with deformation zones formed by its large and small faces, made at angles of inclination to the longitudinal axis of the drawing channel and having corresponding compression sections, characterized in that the angle of inclination of the small deformation face zone to the longitudinal axis of the drawing channel exceeds the angle of inclination of the large face to the longitudinal axis of the drawing channel, the length of the compression section of the deformations translational large area faces confining portion greater than the length of the deformation zone of small facets.