Title of Invention

METHOD FOR THERMO MECHNICAL TREATMENT OF SPRING STEEL

Abstract

A method for thermo mechanical treatment of spring steel comprising the heating up of spring steel bars in an induction system (1) to a temperature between 900°C-1100°C at the rate of temperature rise 130K/sec. The steel bars are passed to a compensation furnace 2, by means of roller table (6), a compensation of the heated temperature of the bars takes place over a period of 15 sec to achieve a temperature gradient over the length is 4K. The steel bars are passed to a holding furnace (3) by means of roller table (7) to achieve a constant temperature. The round steel bars are deformed in a diagonal rolling system (4). After coming out of the diagonal rolling system (4), the rolled bars enter into a post-heating furnace (5) in which they are subjected to post-heating (900-1100°C) to ensure complete static re-crystallization. The diagonally rolled bars are transported by means of a roll table (8) from furnace (5) again transported with the help of a hand over roll table (9). The bars are handed over by hand over roll table (9) to a lift table (10) and from there reach into the CNC-winding bench (11) where the winding into the helical springs take place. The helical springs are transported to a hardening basin (12) wherein they are quenched. There after the hardened helical springs are subjected to a tempering treatment.

Full Text

Method for Thermo Mechanical Treatment of Steel Description The invention pertains to a method for producing helical springs or stabilisers of steel according to the introductory part of claim 1. The document DE 43 40 568 C2 gives a method for continuous annealing of steel wire, which contains the following steps: Quick heating of the wire to a temperature in the austenitic range with a velocity between 85 and 100°C/sec; Holding the steel wire in the austenitic range for a period 10 to 60 secs.; Quenching of the steel wire up to the room temperature with a velocity >80°C/sec; - Quick heating to tempering temperature with a velocity of 85 to 95°C/sec; Holding at the tempering temperature for a period of 60 to 100 sec; Cooling the wire with the usual the velocity of >50°C/sec, generally used for water cooling. Between step 2 and 3 the wire can be tightly rolled above the Ac3 temperature. The wire is first subjected to an oval pass, then round-rolled in a second pass and subsequently drawn through a calibrating nozzle. The document DE 195 46 204 C1 describes a method for producing high-strength objects from heat-treatable steel and application of this method for producing springs. The steel with (in mass-%) 0.4 to 0.6% C, up to 1% Si, up to 1.8% Mn, 0.8 to 1.5% Cr, 0.03 to 0.1% Np, 0 to 0.2% V, and the rest iron, should be treated as follows: The preliminary material is subjected to solution treatment in the austenitic range at temperatures of 1050 to 1200°C; Immediately thereafter the starting material is heat-deformed in a first step at a temperature above the re-crystallization temperature; Immediately thereafter the starting material is heat-deformed in a second step at a temperature below the re-crystallization temperature, however above the Ac3 temperature; - The rolled product is then kept at a temperature above the Ac3 temperature for carrying out further deformation and treatment processes; and thereafter Cooled to below the martensite temperature, and Then tempered. The document DE 196 37 968 C2 gives a method for high-temperature-thermo- mechanical production of spring plates for plate springs and/or plate spring deflectors, which is based on a two step thermo mechanical parabolic spring production. The method is based on the following steps: The parent material is heated to austenitisation temperature with a heating velocity between 4°C/sec. and 30°C/sec; - The austenitisation temperature lies at 1100 ? 100°C, - The material is cooled from the austenitisation temperature to the temperature of the first rolling stage with a cooling velocity between 10°C/sec. and 30°C/sec. First, in a first rolling stage, preliminary rolling is done in one or more passes at a temperature of 1050?100°C with a shape change between 15% and 18% which is not constant over the length of the spring plate; - Cooling is then done from the temperature of the first rolling stage to the temperature of the second rolling stage with a cooling velocity between 10°C/sec. and 30°C/sec. In the second rolling stage, finish-rolling is done at a temperature of 880 ? 30°C with a shape change between 15% and 45% which is constant over the length of the spring length, in one or more passes, with rollers which can be adjusted under load. Finally, the document DE 198 39 383 C2 publishes a method for thermo-mechanical treatment of steel for spring elements subjected to torsion, whereby the parent material is deformed to a temperature above the re-crystallization temperature and subsequently at such a temperature above the re-crystallization temperature, in at least two deformation stages, in such a way that one obtains a dynamic and/or static crystallization of the austenite. The thus re-crystallised austenite of the deformed product is quenched and tempered. One should use a silicon-chromium-steel with a carbon content of 0.25 to 0.75%, which is micro-alloyed with vanadium or some other alloy element. The methods existing in the state-of-the-art technology for thermo-mechanical treatment of objects consisting of steel are mainly based on multi-stage deforming steps, whereby multiple cooling/heating of the parent material is necessary in order to later create parameters which can be adjusted in the end product. It is the task of this invention to present a method for producing helical springs or stabilisers of steel according to the introductory part of claim 1, which allows a specific improvement in the characteristic parameters directed at the loading profile of the end product. This task is fulfilled by means of a method having the features mentioned in patent claim 1. Advantageous extensions of the method are described in claims 2 to 24. According to the method as per the invention, the parent material is first heated up to a temperature above the re-crystallization temperature and subsequently a temperature compensation takes place over the entire bar length. In addition to that, the heating temperature of the bar is kept constant almost up to the point of running into the roller gap. With these working steps one endeavours to achieve a homogeneous texture structure of the bar as far as possible, over its length as well as over its cross-section before entry into the roller gap; this is advantageous for the subsequent deformation process. On account of the process-specific characteristics of diagonal rolling and by specifically laying down the rolling parameters, a pre-defined torsion of the material sets in in the edge of the bars and a deformation gradient sets in over the bar cross-section. As during diagonal rolling the deformation direction runs inclined to the axis of the rolled stock and the maximum of deformation lies in the edge region of the bars, the deformation-caused texture extension in this zone is particularly strongly pronounced and the structure alignment runs corresponding to the deformation direction, also similarly inclined to the axis of the rolled stock. After exceeding the critical deformation degree the dynamic re-crystallization process gets intensively reduced in this edge zone, so that one can identify a slope of the re-crystallization degree over the bar cross-section from outside to inside. Following the deformation process, during post-heating above Ac3 the static re-crystallization is completed, which leads to the formation of fine-grained austenite, particularly in the edge zone. After hardening and subsequent tempering, the edge zone is marked by fine martensite-structures of high stability. The invention presents significant advantages as compared to the known solutions in the state-of-the-art technology. In the result of the combination of a specific, single-step deformation with the help of diagonal rolling and a heat treatment attuned to it, the treated bars reveal a stability profile over their cross-section which reaches its maximum values in the edge regions. The torsion direction of the structure in the edge region of the round bars caused by the diagonal rolling corresponds to the main tension direction of a component subjected to torsion, and the characteristic feature of the bars resulting from it thus reveal optimum pre-requisites for their application, especially in the spring industry. The structure distribution over the bar cross-section caused by the method as per the invention leads in the completely treated round bars to a characteristic profile which is adequate for the tension profile over the bar cross-section for bending and torsion load. As only one deformation step is required for forming these advantageous stability effects, and the subsequent treatment steps are mainly carried out under heat, consequently only one heating up process is required for the parent material, which as a result of this method leads to significant savings in energy and time. Therefore, as compared to the known method, the method as per the invention not only ensures an improvement of the loading-oriented stability and toughness properties of the final product, but beyond that also ensures economic advantages on account of the minimum number of process steps. The parent material in the form of round bars is inductively heated up at a heating velocity of 100 to 200 K/sec. to a temperature between 700 and 1100°C. Subsequently a compensation of the heating temperature of the bar takes place over its length for a period of at least 10 sec. In this way it is ensured that a temperature difference over the bar length does not exceed 5 K. With the help of a suitable post-heating unit the heated up temperature of the bar is kept constant till it runs into the roller gap. The re-modelling itself takes place by means of diagonal rolling in a step, in which the bars run through the roller gap while remaining straight. Depending on the material of the parent material, the re-modelling preferably takes place in a temperature range of 700 - 1150°C. The diameter ratio initial diameter/final diameter is selected in such a way that the diagonal rolling of the bars take place with an average stretching degree ? of greater than 1.3 and the maximum re-formation is at least ? = 0.3. By specific adjustment of the rolling parameters, like e.g. rolling rotation speed and feed velocity, and by means of the special selection of roller contours with specific angular relationship, it can be achieved that the maximum deformation lies in the edge region between 0.65 and 1.0 of the diameter of the bars and that the desired deformation gradient sets in over the bar cross-section. The diagonal rolling process is preferably controlled in such a way that, in the rolled stock a maximum local temperature increase of 50K is not exceeded. On account of the deformation, after exceeding a critical deformation degree, dynamic re- crystallization processes take place, which are more strongly pronounced in the edge zone on account of maximum deformation than in the core region of the bars. The specifically targeted influence of the formation of a deformation gradient over the bar cross-section brings in the effect, that already in the core of the dynamic re- crystallization, the first signs of a differentiated structure distribution over the rolled stock cross-section stets in. Thus in metallographic investigations in bars rolled as per the invention, one can find in the re-crystallized condition, a significant reduction in the share of austenite crystals from the edge zone in the direction of the core region. The differentiated structure formation over the rolled stock cross-section is also additionally strengthened by a typical property of diagonal rolling. As during diagonal rolling the deformation direction runs inclined to the rolled stock axis, one particularly obtains in the edge region of the rolled stock a conspicuous structure-stretching on account of higher deformation. Corresponding to the deformation direction the structure also runs inclined to the axis of the rolled stock and leads to torsion of the material in the edge region. In the process sequence as per the invention, the torsion direction of the structure in the edge direction of the bars is 35 to 65 degrees with respect to the longitudinal axis of the bar and thus corresponds to the main tension direction of a component subjected to torsion. In the shown method of single-step diagonal rolling, the bar to be rolled runs through with its entire length through a roller gap with constant roller gap geometry. This method is selected when bars with uniform diameter over the bar length are supposed to be produced. The method as per the invention further allows an alternative process variant, in which the roller gap geometry in the operating condition is changed during the passage of the bar to be rolled through the roll gap. This flexible working method is realized with the help of a diagonal rolling set-up, whose rollers, if required, can be displaced during deformation in axial and/or radial direction. In this way the method as per the invention allows production of round bars with variable diameter over the bar length. Immediately after coming out of the rolling set-up the diagonally rolled bars are subjected to post- heating at a temperature of above Ac3. The post-heating is done in such a way that the temperature difference over the length of a bar is limited to 5K. Depending on the later purpose of application, the diagonally rolled bars post-heated to re-crystallization temperatures are either wound into a helical spring under heat or bent to form a stabiliser. The wound or bent components are subsequently hardened and then tempered. Bars which are foreseen for production of torsion-bar springs, after post-heating are mechanically treated at their ends in cold condition, subsequently heated above Ac3, quenched and tempered. Macro-investigations on the finished process bars reveal a typical structure distribution over the bar cross-section as a result of the combination as per the invention of diagonal rolling and heat treatment. The immediate edge zone reveals fine-grained a martensite structure of high stability. The edge region reveals a continuous structure-stretching inclined to the bar axis, whose torsion direction corresponds to the main torsion direction of a component subjected to torsion. The pearlitic-martensitic mixed structure of the core zone is coarser-grained than the structure in the edge region and does not reveal any torsion occurrences. For setting optimum toughness and stability parameters in the finished product, the parent material used for the method as per the invention is round bars made of spring steel, preferably silicon- chromium-steels with carbon content steels could be micro-alloyed with vanadium or Niob. The object of the invention is depicted in the drawing on the basis of a design example and is described below. The only figure shows the principle structure of a line production for thermo mechanical treatment of round steel bars made of a silicon-chromium-steel as per the invention. The bars to be treated are heated up in an induction system to a temperature above the re- crystallization temperature, whereby their structure is austenitised . In the present example the round steel bars are heated up at a heating velocity of 130K/sec. to a temperature of 980°C. In the compensation furnace 2 connected to the induction system 1, a compensation of the heated temperature of the bars takes place over a period of 15 sec, so that the temperature graph over the length of the bars reveals a gradient of 4K. In this condition, the now uniformly tempered round steel bars are introduced into a holding furnace 3 in order to retain their temperature constant till they enter into the roll gap. The heated up bars are transported into the compensated furnace 2 and also in the holding furnace 3 by means of roll tables 6 or 7. In a diagonal rolling system 4 the round steel bars heated up to a temperature of 980°C are deformed in the rolling stage. In doing so, the diameter ratio initial diameter/final diameter is selected in such a way that one can work with an average stretching degree A. = 1.5 and that the maximum deformation amounts to at least ? = 0.3. With the help of the specifically targeted setting of rolling parameters, like e.g. the rolling rotation speed or the feed velocity, and by means of special selection of roller contours with specific angular relationship, the maximum deformation is achieved in the edge region between 0.65 and 1.0 of the diameter of the bars and thus the desired deformation gradients sets in over the bar cross-section. The rolling parameters are attuned one another in such a way that a maximum local temperature increase of 50K is not exceeded in the rolled stock on account of the deformation process. The deformation direction running inclined to the rolling axis during diagonal rolling effects a pronounced structure-stretching in the edge regions of the rolled stock on account of higher deformation. Corresponding to the deformation direction, this structure-stretching similarly runs inclined to the axis to the rolled stock and leads to a torsion of the material in the edge region of the bars. In the process sequence of the method as per the invention, the torsion direction of the structure, with respect to the longitudinal axis of the bars, amounts to 35 to 65° and thus corresponds to the main tension direction of a component subjected to torsion. After coming out of the diagonal rolling system 4, the rolled bars enter into the post- heating furnace 5 connected to the system, in which they are subjected to post-heating above the Ac3 temperature in order to ensure a complete static re-crystallization. Transportation of the bars through the post-heating furnace 5 takes place with the help of a roll table 8. After leaving the post-heating furnace 5, the diagonally rolled bars are further transported with the help of a handover roll table 9. From this handover roll table 9 the bars are fed to the further foreseen processing steps. Fig. 1 shows a production line for producing wound helical springs. Thereafter the bars are handed over by the handover roll table 9 to a lift table 10 and from there reach into the CNC-winding bench 11, where the winding into the helical springs takes place under heat after re-crystallization. After the winding sequence, the bars now wound into helical springs, are transferred to a hardening basin 12, in which they are quenched and their structure is transformed to martensite. Subsequently the hardened helical springs are subjected to a tempering treatment which is not shown. List of reference symbols 1. Induction system 2. Compensation furnace 3. Holding furnace 4. Diagonal rolling system 5. Post-heating furnace 6. Roll table 7. Roll table 8. Roll table 9. Handover roll table 10. Lift table 11. CNC-winding bench 12. Hardening basin WE CLAIM 1. Method for thermo-mechanical treatment of steel, where by the parent material is heated up to a temperature above re-crystallization temperature, held and stabilized at austenitic temperature, then deformed and subsequently quenched to martensite and tempered, wherein the parent material is round bars whose heated up temperature is equalized over the bar length and which are subsequently deformed through diagonal rolling, keeping them straight, in such a manner that a pre-defined torsion of the material and a desired deformation gradient over the cross-section is achieved in the edge region, and whereby after exceeding a critical deformation degree, dynamic re-crystallization processes take place and subsequently post-heating above Ac3 takes place, and then the bars are hardened and tempered. 2. The method as claimed in claim 1, wherein the material is heated up at the rate of 100-400 K/sec. 3. The method as claimed in claim 1 or claim 2, wherein the parent material is heated up to a temperature between 7000 and 1100°C. 4. The method as claimed in one or more of the claims 1 to 3, wherein the heating up takes place in an induction heating means. 5. The method as claimed in one or more of the claims 1 to 4, wherein the compensation/equalization of the heated temperature of the takes place for at least 10 sec. 6. The method as claimed in one or more of the claims 1 to 5, wherein the temperature difference over a bar length does not exceed 5K. 7. The method as claimed in one or more of the claims 1 to 6, wherein the heated up temperature of the bar is kept constant almost up to the time it runs in between the rollers. 8. The method as claimed in one or more of the claims 1 to 7, wherein the deformation through diagonal rolling is carried out in one step. 9. The method as claimed in one or more of the claims 1 to 8, wherein the diagonal rolling of the bar takes place with an average stretching degree ? is at least 1.3. 10.The method as claimed in one or more of the claims 8 or 9, wherein maximum deformation in the edge region lies between 0.65 and 1.0 of the diameter of the bar, and ? however is at least 0.3. 11. The method as claimed in one or more of the claims 1 to 10, wherein during diagonal roiling a maximum local temperature increase of 50K should not be exceeded. 12. The method as claimed in one or more of the claims 1 to 11, wherein torsion direction of the structure in the edge region of the round bar corresponds to the main tension direction of a component subjected to torsion. 13. The method as claimed in claim 12, wherein the torsion direction of the structure in the edge zone, with respect to the axis of the round bar, amounts to 35-65°. 14.The method as claimed in one or more of the claims 1 to 13, wherein the structure distribution over the cross-section of the completely treated round bar leads to a property profile which matches the tension profile over the cross-section in case of bending load and/or torsion load. 15.The method as claimed in one or more of the claims 1 to 14, wherein the diagonal rolling is conducted in the temperature range of 700-1100°C. 16. The method as claimed in one or more of the claims 1 to 15, wherein the rollers of the diagonal rolling system are displaced in axial and/ or radial direction during deformation and the round bars are produced over the bar length with a variable diameters. 17.The method as claimed in one or more of the claims 1 to 16, wherein following the diagonal rolling, a post-heating above Ac3 takes place where the temperature difference over the bar length is limited to maximum 5K. 18. The method as claimed in one or more of the claims 1 to 17, wherein springs steel is used as starting material. 19.The method as claimed in one or more of the claims 1 to 17, wherein a silicon-chromium-steel is used as starting material. 20.The method as claimed in one or more of the claims 1 to 17, wherein a micro-alloyed steel is used as starting material. 21. The method as claimed in one or more of the claims 1 to 20, wherein the diagonally rolled, almost straight bar, is wound into a helical spring. 22. The method as claimed in one or more of the claims 1 to 20, wherein the diagonal-rolled bar, approx, straight bar, is bent into a stabiliser. 23. Method as claimed in claims 21 or 20, wherein the diagonal rolled bar as torsion bar remains almost straight and its ends are treated. Dated this 22nd day of August 2005 A method for thermo mechanical treatment of spring steel comprising the heating up of spring steel bars in an induction system (1) to a temperature between 900°C-1100°C at the rate of temperature rise 130K/sec. The steel bars are passed to a compensation furnace 2, by means of roller table (6), a compensation of the heated temperature of the bars takes place over a period of 15 sec to achieve a temperature gradient over the length is 4K. The steel bars are passed to a holding furnace (3) by means of roller table (7) to achieve a constant temperature. The round steel bars are deformed in a diagonal rolling system (4). After coming out of the diagonal rolling system (4), the rolled bars enter into a post-heating furnace (5) in which they are subjected to post-heating (900-1100°C) to ensure complete static re-crystallization. The diagonally rolled bars are transported by means of a roll table (8) from furnace (5) again transported with the help of a hand over roll table (9). The bars are handed over by hand over roll table (9) to a lift table (10) and from there reach into the CNC-winding bench (11) where the winding into the helical springs take place. The helical springs are transported to a hardening basin (12) wherein they are quenched. There after the hardened helical springs are subjected to a tempering treatment.

Documents:

1678-KOLNP-2005-(01-03-2012)-CORRESPONDENCE.pdf

1678-KOLNP-2005-(01-03-2012)-PA.pdf

1678-KOLNP-2005-CORRESPONDENCE.pdf

1678-KOLNP-2005-FORM 27-1.1.pdf

1678-KOLNP-2005-FORM 27.pdf

1678-kolnp-2005-granted-abstract.pdf

1678-kolnp-2005-granted-claims.pdf

1678-kolnp-2005-granted-correspondence.pdf

1678-kolnp-2005-granted-description (complete).pdf

1678-kolnp-2005-granted-drawings.pdf

1678-kolnp-2005-granted-examination report.pdf

1678-kolnp-2005-granted-form 1.pdf

1678-kolnp-2005-granted-form 18.pdf

1678-kolnp-2005-granted-form 2.pdf

1678-kolnp-2005-granted-form 26.pdf

1678-kolnp-2005-granted-form 3.pdf

1678-kolnp-2005-granted-form 5.pdf

1678-kolnp-2005-granted-letter patent.pdf

1678-kolnp-2005-granted-reply to examination report.pdf

1678-kolnp-2005-granted-specification.pdf

1678-kolnp-2005-granted-translated copy of priority document.pdf