DE4233462C2 - Process for increasing the tensile strength and toughness of martensitic steel wires and device for carrying out the process - Google Patents

Description

The invention relates to a method according to the preamble of claim 1 specified type and a device for Execution of the procedure.

Such a process is from the magazine "wire 38" (1987) 8/9, pp. 681-684. In the published there Article states that one can make a significant improvement in the Performance characteristics of the finally tempered components in the Manufacturing technology achieved in that the parts later Lich cold-formed. The article is also to be found take that in fastening technology z. B. is known that the cold rolling or rolling of the screw thread after the remuneration to double the durability of the Screw leads. This advantage of the final shaping is achieved one with all wire products made from the tempered wire Maraform can be manufactured. Maraform wires are usually used cold drawn. The procedure is for spring wires with tension strengths of about 500 to 1800 N / mm² applied.

From the magazine "Draht 43" (1992) 3, p. 291, it is be knows that the work-hardened, austenitic steels Long-term stability is inadequate, apart from the aniso tropie and the expected Bauschinger effect. In addition is The article suggests that it aims at martensite hardening the spring steel is in a temperature range from 300 to 400 ° C with high strength and sufficient toughness very good long-term properties when at rest to reach.  

Spring steel wire is used for spring elements like them for example as helical compression springs in the automotive industry are needed.

The present invention is therefore based on the object the properties, especially the residual stresses Tensile strength and toughness, the well-known martensiti to improve spring steel wires and an economical and process which can be carried out with little effort, and the necessary devices for performing this method to accomplish.

This task is characteristic for the process Features of claim 1 and for the device the characterizing features of claim 7 solved.

Further features of the invention emerge from the sub claims.

The invention thus uses the solution to the present problem the plastomechanical potential of the steel wire in one hitherto unknown extent in such a way that from a mar spring which is converted very uniformly as far as possible steel wire with tempering strengths over 1300 N / mm² is gone and to this basic strength that up to 2000 N / mm² and above, a significant cold ver Percentage of consolidation through a multi-axis cold formation is added.

According to the invention, the steel wire is therefore of a ring shape with the usual radius of curvature of about 500 mm going massive first and then different cold rolled.  

As extensive tests have shown, this causes usual wire forming by cold drawing a loss of advantages mentioned above, once the initial strength of the steel wire exceeds the order of 1300 N / mm² and the wire diameter becomes larger than about 3 mm. This The trend is still grave with martensitic spring steel wire render than with the patented carbon steel. From these and Similar experiences could now be found who that the cold formability of the martensitic steel wire surprisingly under certain conditions gets higher as its strength increases.

The basic condition for the existence of high cold forming Availability at high spring wire strengths is that in the structure of the steel wire an extremely complete homogeneous slat Martensite conversion is discontinued; Procedure the one enable such z. B. in DE 28 30 153 C2 and DE 30 35 032 C1 described and z. T. industrial utilized.

With such martensitic spring steel wires, the plastic and mechanical potentials in a favorable Ratio, so that the base for both cold forming very high strength as well as for the presence of techno logically necessary material toughness in the finished cold shaped spring is given.

Therefore, starting from such a martensitic Spring steel wire, by the measures according to the invention suggested high dynamic spring loads, extreme small force tolerances in large series production, maximum Material utilization and weight reduction as well as smallest possible relaxation values attainable.  

The present invention is based on the knowledge that that a significant improvement in operational properties of feathers primarily by creating useful properties tensions on the way of appropriate plastic cold forming tion of ge used for the production of such springs to reach long spring steel wire is in the way that the minor work hardening effects of the afterbe State-of-the-art handling procedures e.g. B. bullet radiate and set in particular for the particular their use of a spring intensified and about that So far unattainable measure beyond the interests of be adapted to the respective application.

By means of the device for carrying out the The first step is to apply the process long martensitic spring wire high strength, in particular a high 0.2 proof stress (Rp 0.2), by maintaining or even increasing toughness property (fracture constriction Z) is cold rolled.

The well-known classic process of cold drawing steel wire is excluded for this, which is due to the fact that the Cold drawing technology in the case of martensitic steel wires Feasibility limit of a rational manufacturing at a Wire strength of more than 1300 N / mm² and a wire diameter knife of more than 3 mm reached. Beyond these limits fractures and cracks occur more often. Under the simplified Assumption of the validity of Hooke's law of elasticity would the theoretical tension on an outer bend line of a round wire with a diameter of 10 mm at one Radius of curvature of 1000 mm is already 2000 N / mm²; the Material strength is usually at this stress level already exceeded. Whether a material separation on this Surface line of the base wire takes place in such a case, depends on the extent to which the structural condition of the steel wire  is ductile and what state of internal stress in the area of Surface line prevails to the resulting curvature counteract tension by plastic shape change. At increasing curvature, larger wire diameter and alder The cold drawing tensile stress creates a no longer pla limit state to be counteracted, the drawing technology gisch also due to high breakdown values of about 40% to material separations from the wire surface walking leads. Above all results from the addition of the draw tension between the drawing tool and the drawing drum or the trigger device and the compressive stresses in the forming zone of the drawing tool as well as the curvature pull and pressure tension with and without guide rollers under tension standing drawing line an inhomogeneous total stress state.

If the cold-drawing tension of the tensile force is eliminated, it relaxes the critical limit state and you can get that on high strength ductile martensitic spring Form wire break and crack width.

The inventive use of the known cold rolling technology on the high-strength martensitic spring steel wire was found as a means of tensioning Beat the round wire to be cold formed at one Diameter d <3 mm and Rm <1300 N / mm² and thereby the Origin of the voltage peaks and the one-dimensional Strain hardening in the wire axis direction avoid.

The procedural step that would otherwise have to be carried out separately Straightening process of the wire in the cold rolling process to inte Grieren has the further advantage of being initially torsion-free Reshaping the spring wire as caused by that Preparing and cold drawing with the irregular bending and Torsional loads and predominantly quasi-one-dimensional  Solidification is not possible. In cold rolling, the Formability no longer due to the "drawn material" wire itself Forming zone, but gently from the outside through the pairs of rollers each with larger printing and forming areas than one "Drawing die" transferred. The cold rolling of high strength mar tensitic steel wire carries - like comparative tests Wires that have been tempered according to the state of the art are - show - a simultaneous increase in Festig speed and toughness, while this only applies to cold drawing The case is if under gentle industrial not durable Conditions in multiple or subsequent train mode of operation cold is pulled.

For the use of martensitic steel wire as spring wire However, a basic requirement is that both strength as well as the toughness have high values. Desirable are z. B. for high-voltage helical compression springs values for Rm around 2000 N / mm² and Z around 40% and above.

Another basic mechanical size caused by the cold whale zen is influenced far more than in cold drawing, is Yield strength, expressed e.g. B. by the 0.2 proof stress Rp 0.2. This size is very important for the spring properties increases up to about 99% of the value of the tensile strength without that the equally important constriction decreases.

The invention can no longer - as with cold pull - primarily axially aligned cold forming structure of the cold rolled wire martensitic structure base high Strength better than previously the ideal plastomechanical State of maximum elasticity and high plasticity closer to the respective steel wire potential. To According to the invention, the forming parameters in cold rolling are each according to the degree of the combination of the basic remuneration strength and the total work hardening matched to each other.  

The cyclic cold forming possible in all in cold rolling spatial directions caused a quasi-isotropic offset structure and thereby favorable conditions for the Slatted martensite, which then has the sliding mechanisms in the structure dominate in such a way that it affects the fracture constriction major disadvantages of the original grain size in the Austenite will be pushed back.

Experiments have shown that this effect is less of the hardening strength as the structural condition of the steel wire tes depends as long as the strength according to the invention in the range the spring application, that is over 1300 N / mm². At low Lower initial strengths lead to a decrease in fracture lacing, as there is increased carbide in the structure of the steel there are divorces for lower strengths necessary tempering with higher temperature arise. This Carbides prevent sliding and reduce plastic ver shaping potential of steel. This also results in the Condition for the martensite morphology of a lath marten sits for which the carbon content of the steel wire is not may be higher than 0.50% to 0.55%.

The cold forming on which the present invention is based in many directions the goal already mentioned has To cause residual stresses in the spring steel wire, that are useful to the spring under load. So the resistance moments of such screws compression spring significantly increased. Since helical compression springs in essential to torsion, it is a glowing that it comes down to the thrust Stress strength values of the used for their manufacture To increase steel wire.  

This is achieved according to the invention in that the steel wire at Cold rolling from roll stand to roll stand specifically twisting cold formed, the degree and direction of the gate depending on the spring construction and the application judges.

The step according to the invention is also advantageous in that the steel wire coming from the last mill stand directly in a diameter equalizing, rotating bearing pressure and the forming nozzle is pressed in. This printing and forming The nozzle is used to ensure that the spring is used torsional residual stresses, the sense of rotation and the amount can be adjusted accordingly.

It works in the same way instead of printing and forming nozzle used rotating roller, by means of which also the boundary layer forming mentioned above can be carried out is.

Due to the above-mentioned surface layer forming at the same time the roundness of the steel wire to one cold drawn steel wire increased.

Another advantage is that the plastomecha niche forming of the steel wire prepare the possibility tet, by a thermal low-temperature treatment by about 250 ° C the shear elastic limit, d. H. so that z. B. the Increase the shear yield point.

Such heat treatments to remove the "Bauschinger" called recovery-relief effect are known and for example in connection with the martensitic structure in the magazine "Draht 38" (1987) 8/9, pp. 681 to 684. Unlike the up The original possibility and way of looking at it is with  present invention is not primarily about Bauschinger effects with tensile stresses and for high yield strength values, but because the torsional moment of resistance of the steel wire thanks to increased Edge shear stress resistance to avoid premature flow of the boundary layer areas of the spring surface increase under vibrating load. Because this is dangerous areas due to the geometry of helical compression springs lying on the inside of the spring helps, like experience shows the usual measure by shot peening to Druckfe The surface is only slightly increased.

Through the method step according to the invention, for the purpose of Er Increasing the elastic properties - especially the that of the torsional load - the steel wire of a heat Aftermath during or immediately after cold working - Cold rolling plus twisting - these are subjected to Properties significantly increased.

The improvement of the plastomechanical properties of the However, spring wire requires that the texture the same, in particular in the peripheral zone area and on the Surface is as flawless as possible. Material separations in Form of cracks - also fine cracks and 40 µ crack depth from the Surface and edge decarburization or decarburization are according to countered the prior art in that the spring roller in many cases cutting wire up to 10% of the Surface weight deduction by "peeling" receives. Stay with a cold-wound helical compression spring namely the circumferential grooves of the rotary peeling, also one Rotary grinding, obtained while the same is being thermoformed Austenitizing feather largely imparted sharpness ren.  

For the pre-invented according to the invention as a starting material struck spring steel wire, on the other hand, there is a surface deduction before cold rolling without cross marks in such a way that a hydrodynamically acting system unites the surface abrasive transport of hard materials contrary to processing direction of the steel wire causes.

By reshaping the martensitic according to the invention round output wire during cold rolling to an im Cross-section of inconsistent cold-rolled wire using the program controlled additional roll stands at a suitable location within the cold rolling mill according to specified lengths and profiles Interfering with the rolling process, there is the advantage of a simple, reproducible production, in particular also by the measure that the cylindrical-conical cold forming with a round cross-section the cold rolling rotie rend is carried out around the wire axis.

The spring wire according to the invention with an inconsistent diameter is used for production using a conventional wind car mats of helical compression springs with a small block height, so like this called conical compression springs, the spring in the vehicle little take up space. This is now no longer necessary in this production hitherto common peeling and the associated disadvantages of material waste and it will cut the cold ver strengthening fiber and an impairment of the internal stresses as well as a newly created martensite, which without immediately to be relaxed on the surface of the spring wire Fine cracks leads, avoided.

The invention is described below with reference to a production line with several exemplary embodiments of devices according to the invention for carrying out the method according to the invention which are more or less schematically illustrated in FIGS. 1 to 5.

In detail show:

Fig. 1 is a side view of a production line belonging to the device for directing a massive round spring steel wire by means of a straightening roll pair with just subsequent guide roller sets,

Fig. 2 is a view of an arrangement of two cold rolling mills to each other and of guide rollers between these Kalzwalzgerüsten as part of the production line to the twisting of a round spring steel wire during the cold rolling,

Fig. 3a shows an arrangement for equalizing the edge of a spring steel wire layer forming by a rotating pressure nozzle as part of a finished cold rolling stand of the production line,

FIG. 3b, an arrangement for equalizing the edge of a spring steel wire layer forming with a rotating cross-rolling apparatus as part of a finished cold rolling mill of the production line,

Fig. 4 is a plan view of an illustration of a continuous low-temperature heating arrangement between the cold rolling end stand or the edge layer forming according to FIGS . 3a and 3b and the reeling of the shaped spring steel wire, and

Fig. 5 shows an arrangement of a hydrodynamically acting hard material-liquid mixture system for longitudinal machining of the surface of a spring steel wire between the solid straightening according to Fig. 1 and the cold forming by cold rolling within the production line.

At the beginning of the production line, as shown in FIG. 1, a martensitic spring steel wire 1.2 elastically bent back from a wire ring 1.1 runs directly into a straightening roller pair 1.3 of a straightening roll stand, in which the spring steel wire 1.2 is quickly cold extruded up to the core and thereby massively straightened, once it is in a straight direction e.g. B. is guided by guide rollers 1.4 . This type of straightening according to the invention differs from the known methods in that it does not perform a cyclically elastic plastic reversal of tension.

In Fig. 2 an arrangement for twisting the spring steel wire during cold forming by cold rolling is shown, the cold rolling stands 2.1 are adjustable in the direction of the arrow with respect to the longitudinal axis of the spring steel wire 1.2 and pivotally mounted while the rollers of the rolling mill 2.2 are mounted relative to each other in the direction of arrow . Likewise, the guide rollers 2.4 and 2.5 are adjustable in the direction of the arrow.

With the help of this arrangement, the spring steel wire 1.2 torsional residual stresses are embossed into a helical compression spring even before the later winding of the spring steel wire 1.2 , the desired swirl direction and the actual torsion and thus the desired torsional residual stress being produced by a defined setting of rollers and guide rollers ( 2.1 , 2.2 , 2.4 and 2.5 ) is determined. The twisting takes place in the roller gap, that is, in the plastic state of the martensitic spring steel wire 1.2 , whereby any damage to the surface and in the structure of the spring steel wire 1.2 are avoided.

In Fig. 3a is shown as part of the production line a first arrangement for leveling the outer layer of the spring steel wire 1.2 . For this purpose, a cold-rolling end stand 3.3 of the production line is immediately followed by a rotatably mounted pressure or forming nozzle 3.1 , which can be operated clockwise or counter-clockwise. In addition, an oscillating movement of the pressure nozzle 3.1 is also possible. Through the rotation of the pressure nozzle 3.1 , regardless of the linear speed of the cold-rolled martensitic spring steel wire 1.2 , the desired torsion specified by the cold rolling process is set in accordance with the intended use of the spring steel wire, with a further improvement in the layer layer due to the oscillation movement of the pressure nozzle 3.1, which is also dependent on the speed results.

As shown in Fig. 3b, instead of the pressure nozzle 3.1, the desired improvement in the outer layer and torsion of the spring steel wire can also be brought about by an inclined roller apparatus 3.2 rotating around the spring steel wire 1.2 , the angle of which to the spring steel wire 1.2 is adjustable. Depending on the setting angle of the cross roll apparatus 3.2 in the direction of the arrow and its speed, the torsion predetermined by the cold rolling process is set accordingly. The cross roll 3.2 is positioned around or parallel to the axis of the wire 1.2 oscillating or fixed, while the rotation of the rollers depending on the speed of the spring steel wire 1.2 is adjustable.

The degree of cold rolling described, based on the initial cross section of the spring steel wire 1.2, is between 6% and 80% depending on the intended use and the properties of the starting material.

In Fig. 4, an arrangement of a continuous low-temperature heating device in the form of a 4.3 arranged in front of a Haspelvor heating station 4.2 is shown, in which a targeted heating of the spring steel wire 1.2 in the alpha region of the material immediately after the cold rolling process using the already existing forming heat is reached. In this way, an improvement in the edge shear strength of the spring steel wire 1 . 2 scored. The heating station 4.2 works depending on the speed and volume, so that rich can be driven with very narrow Temperaturbe. Of course, this heat treatment can be repeated.

In Fig. 5, a hydrodynamically operating longitudinal cutting device is shown. In order to avoid material separations in the end product caused by surface defects or fine cracks, the martensitic spring steel wires 1.2 are mechanically processed in a downstream countercurrent grinding device 5.1 after the solid device operation according to FIG. 1. This device comprises a tube provided with an inner spiral, through which a hard material abrasive is pressed against the wire running direction 1.5 at high pressure. As a result, a continuous upper surface processing without transverse grooves or without circumferential grooves is achieved, so that there is a significant improvement in quality with such spring steel wires produced helical compression springs and at the same time considerable material savings and increase in economy. As is well known, a normal peeling process results in material loss of up to 10% through machining.

Claims (16)

1. A method for increasing the tensile strength and toughness of martensitic spring steel wire ( 1.2 ) for the production of Federelemen th, characterized in that the martensitic spring steel wire ( 1.2 ) with an initial tensile strength of more than 1300 N / mm² and a diameter of more than 3 mm is initially leveled and then cold rolled between 6 and 80%. 2. The method according to claim 1, characterized in that the cold forming of the spring steel wire ( 1.2 ) by cold rolling an equalizing edge layer forming by a pressure nozzle ( 3.1 ) follows. 3. The method according to claim 1, characterized in that the cold forming of the spring steel wire ( 1.2 ) by cold rolling an equalizing edge layer forming by a wire around the spring steel ( 1.2 ) rotating cross roll apparatus ( 3.2 ) follows. 4. The method according to any one of the preceding and workman surface, characterized in that the spring steel wire ( 1.2 ) is twisted during cold rolling. 5. The method according to any one of the preceding and workman surface, characterized in that the surface of the spring steel wire ( 1.2 ) is ground after straightening. 6. The method according to any one of the preceding and workman surface, characterized in that the spring steel wire ( 1.2 ) is heated after the cold forming and is then wound up. 7. Device for performing the method according to one or more of the preceding claims, characterized by a straightening and cold rolling mill ( 1.3 , 2.1 , 2.2 , 3.3 ). 8. A device for performing the method according to claim 2, characterized in that a pressure nozzle ( 3.1 ) is provided after the cold rolling mill ( 2.1 , 2.2 , 3.3 ). 9. The device according to claim 6, characterized in that the pressure nozzle ( 3.1 ) is rotatably mounted. 10. The device according to claim 7, characterized by an oscillating pressure nozzle ( 3.1 ). 11. A device for performing the method according to claim 3 and / or 4, characterized in that an inclined roller apparatus ( 3.2 ) is provided. 12. The apparatus according to claim 11, characterized in that the angle of the rollers of the inclined roller apparatus ( 3.2 ) to the spring steel wire ( 1.2 ) is adjustable. 13. The apparatus of claim 11 or 12, characterized marked by an axis of the spring steel wire ( 1.2 ) rotating cross roll apparatus ( 3.3 ). 14. The apparatus according to claim 13, characterized by an oscillating movement of the inclined roller apparatus ( 3. 3 ). 15. A device for performing the method according to claim 5, characterized in that a hydrodynamic solid grinding device ( 5.1 ) is provided. 16. The apparatus according to claim 6, characterized in that a continuous low-temperature heating device ( 4.2 ) and a Haspelvorrich device ( 4.3 ) after the cold rolling mill ( 2.1 , 2.2 , 3.2 , 3.3 ) is provided.