Method for producing a semifinished product by using a pearlitic steel, and semifinished product produced by the method

Abstract

A method for producing a semifinished product from a pearlitic steel by annealing is characterized in that the semifinished product is annealed by inductive annealing on spheroidal cementite. The process times are shortened as compared with the conventional method, and the carbides formed are smaller and more finely distributed as compared with the conventional treatment.

Description

[0001] Process for producing a semi-finished product using a pearlitic steel and semi-finished product produced by the process

[0002] The invention relates to a method according to the preamble of patent claim 1.

[0003] Annealing is a heat treatment process for metals that improves machinability and cold formability through annealing. Steel is heated to a temperature between 650 °C and 750 °C until it glows, and then held at elevated temperature for a while until the desired microstructure transformation is achieved. It is then allowed to cool slowly. Annealing reduces the hardness and strength, thereby significantly facilitating further processing steps such as rolling, wire drawing, or punching, especially because it prevents the formation of cracks.

[0004] During annealing, defects such as dislocations are first healed, and stresses in the material are reduced. Then, recrystallization occurs, in which new nuclei form (nucleation) and replace highly strained crystallites. Finally, crystal growth occurs.

[0005] In the case of hypoeutectoid steel, i.e. steel with a carbon content of less than 0.8% C, annealing takes place in the region below the PS line in the iron-carbon diagram (EKD). In the case of hypereutectoid steel, annealing takes place by oscillating around the A1 temperature, i.e. the SK line in the EKD, followed by slow cooling. This produces a state that is sufficiently soft and low-stress for the intended use. Here, A1 designates a holding point or inflection point in the cooling curve, i.e. transformation temperature; the designations Ac1, Ar1, Ac2, Ar2 etc. are also used in the iron-carbon diagram. A stands for "arret", i.e. holding point or inflection point, r for "refroidissement", i.e. cooling, and c for "chauffage", i.e. heating.

[0006] During this annealing process, the state of the microstructure prior to annealing is irrelevant. The focus is solely on changes in elongation at fracture, strength, and hardness. Due to the annealing temperature, the lamellar (strip-like) cementite loses strength and can pursue its goal of becoming a body with the smallest possible surface area (a sphere). Spheroidal cementite forms, which is why it is often referred to as annealing to spheroidal cementite (GKZ annealing). This makes the material easier to form and machine.

[0007] Annealing improves the machinability of steels with a carbon content of more than 0.4 mass percent.

[0008] Annealing is a heat treatment process for metals in which cold formability and machinability are improved through annealing. A special type of annealing is GKZ annealing, which places demands on the microstructure in addition to the mechanical and technological properties. The carbon contained in the steel should be present in spheroidal cementite (FeS 2C) in a ferritic matrix. This annealing treatment further reduces the hardness and strength, which considerably facilitates further processing steps such as rolling, wire drawing and, above all, machining. US 4 604 145 A distinguishes between three types of GKZ annealing, which are explained below using the reference points in the iron-carbon diagram (EKD). These are used to characterize transformation points, referred to below as Ac1, Ar1, Ac2 and Ar2.A stands for “Arret” or stopping point, r for “refroidissement” or cooling and c for “chauffage” or heating.

[0009] The transformation points mentioned can be calculated thermodynamically for a specific chemical analysis. The representation is usually in a time-temperature austenitization diagram (ZTA diagram) (Fig. 1 ), where time is shown logarithmically in seconds on the x-axis, and temperature is shown as a function of time on the y-axis. The thermodynamic states can be read off using a temperature and a heating rate. This is traditionally based on a ferritic-pearlitic initial microstructure. When an Ac1 temperature of 13 is exceeded, the carbides begin to dissolve, and the pearlite transforms into austenite with further temperature increases. When an AC3 temperature of 12 is exceeded, the transformation of the pearlite is complete, and the remaining ferrite transforms into austenite.Only above a "homogeneous austenite" line 11 does fully transformed and homogeneous austenite exist. At an infinitely slow heating rate, curves 12 and 13 coincide with the Ac1 and Ac3 lines in the EKD. Curve 11 can be interpreted as a safety margin for technical processes, where complete austenitization can be assumed.

[0010] The ZTA diagram thus describes the transformations in the non-equilibrium state. A steel with the 100Cr6 alloy is considered a special case in the ZTA; with a hypereutectoid carbon content, curves A1 (13) and A3 (12) are congruent because no ferrite is present.

[0011] GKZ annealing, sometimes also called spheroidal annealing, is usually divided and characterized into three types:

[0012] 1 . Delayed cooling: The steel is heated to temperatures > Ac1 (line Ac1 ), kept at temperatures between Ac1 and Ac3 and then cooled very slowly.

[0013] 2. Isothermal treatment: The steel is heated to temperatures just below Ac1 and kept isothermal for long periods and finally cooled.

[0014] 3. Pendulum annealing: The steel is alternately heated according to method 1 (above Ac1) and cooled according to method 2 (below Ac1). The temperature oscillates around the A1 temperature, hence the name pendulum annealing.

[0015] In practice, hybrids of the three basic principles are often used. However, they all have in common the very long annealing time and the associated high energy consumption. Typical annealing times range between 12 and 50 hours, depending on the chemical composition. DE 10 2004 011 021 A1 discloses a 100Cr6 rolling bearing steel that is soft-annealed in a time-consuming annealing process lasting more than 12 hours to produce a microstructure suitable for further processing with spheroidally formed carbides (GKZ microstructure).

[0016] The very long annealing times are due to the processes involved. Typically, in GKZ annealing, the starting material is in a ferritic-pearlitic (FP) microstructure. The carbon in the steel is present in lamellar form in the form of FesC (cementite) in a ferritic matrix. Annealing in the range of the dissolution temperature (Ac1 ) makes the carbon in the microstructure mobile and strives for the most energetically favorable state with the smallest possible surface area. Diffusion causes spheroidal cementite to form, hence the name GKZ (annealing to spheroidal cementite). This makes the material easier to form and machinable. The tensile strength and hardness decrease, and the elongation at break increases. Rolling bearing steels such as steel with the alloy 100Cr6 can only be machined in this state. This type of heat treatment is generally used for steels with a carbon content of more than 0.4%.

[0017] GKZ annealing is often used for hypereutectoid steels with a carbon content of more than 0.8%, usually through a combination of the three usual process steps:

[0018] Fig. 2 shows a typical time-temperature curve. This consists of heating to temperatures above Ac1 (line 21), an excitation phase (line 22), an isothermal holding phase (line 23) for very long periods, and a final delayed furnace cooling. Typical annealing times are between 30 and 50 hours for technical reasons.

[0019] The initial state of pearlitic steels is characterized by the presence of carbon in the steel as intermetallic phases of the FeSCl type. These particles precipitate in plate-like form upon slow cooling. These carbide plates appear like stripes in microscopic section, hence the term "lamellar pearlite."

[0020] This hard phase within the soft ferrite matrix complicates machining by turning, drilling, and other cutting processes, as the tool must cut through the hard phases. However, if the carbides are in spherical form, the necessary cutting forces and tool wear are reduced.

[0021] Hypereutectoid steels such as the 100Cr6 alloy steel can only be machined in the GKZ temper. State-of-the-art GKZ annealing is performed conventionally in batch-charged furnaces or continuous furnaces as described above.

[0022] US Pat. No. 4,604,145 A demonstrates various possibilities for accelerating the carbide forming process by a factor of 5 to 10. By applying thermomechanical control during the rolling process, spheroidal cementitious annealing has been achieved on various steel grades within significantly shorter times. The starting point is still a pearlitic steel, but warm forming at Ac1 introduces deformation energy into the system. This increases the diffusion rate, and the forming process from pearlite to spheroidal cementite proceeds more quickly.

[0023] DE 692 24 562 T2 describes ways to improve the degree of softening during spheroidal annealing with suitable pretreatment. Starting with hot forming followed by semi-hot forming at temperatures between Ac1 and Ac1 -400°C, a state is created that results in a softer final microstructure after spheroidal annealing. No statements are made here regarding the final microstructure or carbide sizes.

[0024] The metallurgical specifications for microstructure characteristics can be evaluated as follows: Using a metallographic section, the surface area of ​​spherical, embedded carbides, the so-called carbide indentation, is assessed, the so-called degree of indentation in percent. In the initial state, almost all carbides are still in the lamellar state, with a degree of indentation of 0%. If, after annealing, the carbides are still lamellar on 10% of the surface, the degree of indentation is 90%.

[0025] GKZ annealing is used for steels with a carbon content of more than 0.8% (hypereutectoid steels). The aim is to spheroidise the cementite (GKZ = annealing to spheroidal cementite), which is achieved by heating above the transformation temperature Ac1 or by means of pendulum annealing around the transformation point Ac1 in combination with a very long holding phase and slow furnace cooling. GKZ annealing (= Annealing for spheroidized carbides) describes the heat treatment of pearlitic steel grades with a carbon content of 0.3% to 1.2% and admixtures of other alloying elements with the aim of soft annealing with additional requirements regarding carbide formation. In their initial state, pearlitic steels have a microstructure consisting of ferrite with plate-like precipitates of metal carbides of the form M3C, M7C3, or M23C6, the so-called lamellar pearlite.

[0026] This formation of the carbide plates appears like stripes in microscopic cross-section and makes mechanical processing by turning, drilling and other machining processes, as well as forming, more difficult.

[0027] To ensure machinability, an annealing process is used in the state of the art to break up the lamellar structure of the carbides and convert them into a spherical hard phase in a soft ferrite matrix.

[0028] The specifications for the microstructure are defined in a steel-iron test sheet in the form of an illustrated guideline (SEP 1520). A distinction is made between the pearlite content (PA) and the carbide size (CG). Any pearlite content remaining after annealing negatively affects machinability, while coarsening of the spheroidal carbides due to excessively long annealing impedes subsequent hardening processes.

[0029] This type of heat treatment of raw material allows machining of high-carbon steel grades such as 100Cr6 for the ball bearing industry, which are considered almost impossible to machine in their untreated state.

[0030] Currently, spheroidal annealing is carried out in batch-charged furnaces or in very long continuous furnaces. The annealing times range from several hours to several days. In a standard annealing process, for example, the material is heated to 840 °C in a batch-charged furnace at a heating rate of 100 K / h for an excitation phase. After a soaking phase, the furnace undergoes controlled cooling so that a holding temperature of 720 to 730 °C, below the Ac1 temperature, can be achieved for 24 hours. At this holding temperature, the pearlite lamellae fragmentation and ultimately the spheroidal precipitation occur.

[0031] Since the driving force is not the same in every crystal in the microstructure, very long periods of time may be necessary to generate a uniform result in every area across the entire cross-section.

[0032] In addition to the pearlite content, the achieved carbide sizes are also important parameters for characterizing the resulting microstructure. Long annealing times lead to a lower pearlite content, but can produce carbide sizes with a diameter of more than 5 pm (or coarser than 2.3 pm according to the SEP1520 guideline series). According to the invention, the carbides are always essentially spherical; the carbide sizes are always given below as diameters.

[0033] After machining, for example to produce a pump body or a bearing shell for a rolling bearing, the components are subjected to heat treatment; they are usually hardened.

[0034] For this purpose, the components are heated to temperatures between Ac1 and Ac3 (partially austenitized) in order to dissolve only part of the carbon in the austenite.

[0035] The aim is to achieve a defined proportion of residual austenite in the hardening microstructure, or even to avoid it entirely. Hardening temperatures are therefore typically between 840 °C and 880 °C. For coarse carbide formation, higher temperatures are often required, which can lead to distortion during the hardening process. These distortions, in turn, must be corrected through complex post-treatment such as grinding or straightening. A lower hardening temperature has a very positive effect on rework and distortion, and thus on production costs. State-of-the-art annealing techniques for steel with alloy 100Cr6 involve batch-loaded furnaces or very long continuous furnaces. Annealing times range from several tens of hours to several days.

[0036] The process can be described as follows: Heating of the material, for example a rod or a semi-finished steel product to 840 °C, i.e. above the Ac1 temperature, at a heating rate of approximately 100 K / h. After a holding phase for thorough heating, it is cooled to a temperature just below Ac1 and this temperature is held for 24 h. During this holding phase, the lamellar pearlite decomposes and precipitates again in spherical form. These very long annealing times are necessary so that this fragmentation process occurs uniformly at every point and in every intergranular grain. The diffusion rate also depends on the driving force in the form of activation energy. If this process is interrupted too early, areas with a pearlitic structure are often found directly next to areas with incorporated carbides. An example of this is (Fig.3), where area 31 shows the original carbide lamellae (pearlite) and area 32 shows an area with formed spherical carbides.

[0037] The more energy present in the system, e.g., in the form of stresses or high dislocation density, the faster the diffusion processes can proceed. Approaches to increasing diffusion rates by increasing the driving force can be found in US 4 604 145 A using warm forming and also in RU 2 238 338 C1 using cold forming before the actual heat treatment.

[0038] It is the object of the invention to provide a method for producing a semi-finished product from a pearlitic steel by annealing, by means of which complex post-treatment of the semi-finished product after annealing can be avoided.

[0039] According to the invention, this object is achieved as stated in claim 1.

[0040] The present invention provides a method for producing a semi-finished product made of Persian steel with spherically formed carbides and a degree of deformation of up to 100%, ie with a pearlite content of up to 0%, whereby the treatment times are shortened and the energy consumption is reduced.

[0041] A method is created for inductive GKZ annealing, for example of a rolling bearing steel, for example in the form of bar steel.

[0042] The invention provides a method for heat treating steels with the alloy 100Cr6 and other rolling bearing steels with the aim of spherodization or annealing to spheroidal cementite (SCC) using an inductive continuous annealing system. This type of treatment reduces the required annealing times for carbide formation by a factor of ten to one hundred, resulting in significant resource savings. The initial microstructure of ferrite and pearlite is converted via a short-term martensite stage to the initial microstructure of spheroidally formed carbides, which are smaller and more finely distributed than in prior art processes. This reduces the cutting forces during machining and extends tool life.Subsequent heat treatments such as hardening and surface hardening can be carried out at lower process temperatures due to the finer carbide distribution, thereby reducing distortion and resulting rework.

[0043] Inductive heating involves an alternating electromagnetic field inducing eddy currents in a metal workpiece, which are converted into heat through remagnetization losses. The current flow generated in the workpiece heats the metal.

[0044] Thus, a semi-finished product is annealed to spheroidal cementite by inductive annealing.

[0045] Induction annealing is preferably performed on a steel bar. It is advantageous to use a steel bar that is suitable for use in a rolling bearing (rolling bearing steel).

[0046] Advantageously, the carbide formation in the form of carbide size is also smaller than in the state of the art, so that lower temperatures are used during the final hardening than is possible with the state of the art.

[0047] The semi-finished product is annealed to spheroidal cementite by inductive annealing.

[0048] The invention provides a method for inductive GKZ annealing in order to inductively treat a rolling bearing steel as a starting material, for example in the form of bar steel, with heat.

[0049] Advantageous further developments emerge from the subclaims and the embodiments, in particular from the representation in the drawings.

[0050] Induction annealing is preferably applied to a steel bar.

[0051] It is advantageous to use a steel bar that is suitable for use in a rolling bearing (rolling bearing steel).

[0052] The use of chromium steel is particularly advantageous. A steel with an alloy of 100Cr6 is preferred.

[0053] The semi-finished product is preferably inductively heat-treated in a continuous quenching and tempering system. The continuous quenching and tempering system comprises at least two areas for isothermal holding at elevated temperatures.

[0054] Heating rates that allow heating of the semi-finished product at up to 100 K / s are advantageously used.

[0055] In particular, the semi-finished product is advantageously heated to a target temperature of up to 880 °C.

[0056] It is also advantageous if the target temperature is maintained for a period of up to 300 seconds. Preferably, the semi-finished product is quenched after induction heating, for example, with water or cold air.

[0057] It is advantageous to use a quenching speed of more than 50 K / s.

[0058] It is also advantageous to quench the semi-finished product until it reaches a martensitic state.

[0059] Preferably, after quenching, the semi-finished product is heated again to a temperature below the Ac1 temperature.

[0060] A temperature of up to 730 °C is preferably achieved by inductive heating.

[0061] Preferably, this temperature is maintained for a period of up to 500 s.

[0062] It is advantageous to cool the semi-finished product to room temperature after the second heating.

[0063] It is particularly advantageous if the semi-finished product is heated by at least a second inductive annealing.

[0064] In this case, it is also advantageous to heat the semi-finished product again to a temperature below the Ac1 temperature after the second annealing.

[0065] The invention also relates to a semi-finished product produced by the process according to the invention, as claimed by a process according to any one of claims 1 to 19.

[0066] The invention is described in more detail below using exemplary embodiments. In the drawings: Fig. 4 shows a schematic representation of a continuous tempering system for tempering steels.

[0067] Fig. 5 shows a temperature curve for an inductive AC annealing as a function of time and

[0068] Fig. 6a a scanning electron microscope image of

[0069] Microstructure in a conventionally annealed semi-finished product.

[0070] Fig. 6b a scanning electron microscope image of

[0071] Microstructure in an inductively annealed semi-finished product

[0072] In a first embodiment (Fig. 4), a body formed as a bar steel made of a rolling bearing steel with an alloy 1 OOCr6 is inductively treated by heat in a continuous quenching and tempering plant 40.

[0073] 41 denotes a support table for placing long steel products, 42 a unit for inductive heating of steel, 43 a unit for cooling steel, 44 another unit for inductive heating of steel and 45 a unit for collecting the treated steel bars.

[0074] In a first cycle 51 (Fig. 5), the material is inductively heated to temperatures greater than Ac1 but lower than those at which homogeneous austenite forms. The temperature range between 820 and 880 °C is particularly advantageous. The heating rate is selected between 3 K / s and 50 K / s.

[0075] The target temperature is maintained for between 10 and 300 seconds. The material is then rapidly cooled to room temperature (quenching), for example, with water or oil. This hardens the material. Cooling preferably occurs at rates greater than 10 K / s. The resulting microstructure is preferably martensitic. This is followed by a cycle 52 in which the martensitic-hardened material is heated to temperatures below Ac1 and held for a defined time. The heating rate is selected between 3 K / s and 50 K / s, the holding times are between 2 and 15 minutes, followed by cooling in still air to room temperature at less than 10 K / s.

[0076] This cycle 52 is repeated at least once by a cycle 53. The heating and cooling rates may differ from those of cycle 52. The duration of the temperature plateau may also differ from the temperature plateau of cycle 52.

[0077] Example 1 :

[0078] A steel with an alloy 100Cr6, ie, with additional elements in the chemical composition C = 0.96%; Mn = 0.34%; Cr = 1.44%, where C represents carbon, Mn manganese and Cr chromium, and an average hardness in the untreated state of 330 HB and a pearlitic initial structure is subjected to the following treatment route:

[0079] In cycle 51, this alloy is heated to 840 °C at a heating rate of 10 K / s. This temperature is held for 20 s, followed by quenching (hardening) with water to room temperature at a cooling rate of 100 K / s.

[0080] Cycle 52 follows, heating at a rate of 10 K / s to 730 °C. This temperature is maintained for 300 s, followed by air cooling to room temperature. This is followed by a substantially identical cycle 53.

[0081] After completing the process, the achieved hardness is 255 HB, the carbide formation is 100% (pearlite content 0%) with an average carbide size of 250 nm to 500 nm.

[0082] Example 2: A steel of alloy 100Cr6 with a chemical composition of C = 1.02%; Mn = 0.33%; Cr = 1.35% is inductively heated to 860°C at a rate of 10 K / s (cycle 51), quenched with water, and tempered to 730°C in cycles 52 and 53, respectively. The achieved hardness is 270 HV, the carbide formation is 100%, and the carbide size is 250 nm to 500 nm.

[0083] Figures 6a and 6b show micrographs obtained with a scanning electron microscope. The sections were metallographically prepared, etched, and imaged using a scanning electron microscope at 10,000x magnification.

[0084] Fig. 6a shows the spheroidal carbides of a standard spheroidal annealing of a steel with alloy 100Cr6 in a batch-loaded furnace after 730 °C for 40 h. The carbides are spheroidal and have an average carbide size of 500 nm to 1000 nm. The average hardness is 180 HB.

[0085] Fig. 6b shows the spherical carbides from Example 2; the annealing time for cycles 52 and 53 is 600 s in total. The carbide sizes range from 250 nm to 500 nm.

[0086] The advantages of this type of heat treatment are a significant reduction in treatment times and energy savings while achieving equivalent or better results in terms of microstructure.

[0087] A further advantage is the improved machinability of alloy 100Cr6 steel in the inductively AC (iAC) annealed state. The cutting edge of a machining tool circulates the finer carbides during machining instead of cutting through them in the standard process. This has a positive effect on machining efficiency and tool life, despite the comparatively higher hardness.

[0088] Subsequent heat treatment processes are also positively influenced. The finer precipitation of carbides allows for more reliable hardening processes on the machined component. The finer carbides dissolve faster and more evenly, thus shortening the required process times during hardening. The hardening temperature can also be reduced by 10 to 20 K, which in turn has a positive effect on distortion during hardening.

[0089] Precursor material produced in this way is characterized by hardnesses in the range of 240 HB to 300 HB (Brinell hardness). Carbide deposition of up to 100% is achieved; significantly finer carbide precipitates are produced compared to state-of-the-art annealing processes.

Claims

Patent claims 1 . A process for producing a semi-finished product from a Persian steel by annealing, characterized in that the steel is annealed by inductive annealing to spheroidal cementite.

2. A method according to claim 1, characterized in that the steel is annealed for a period of not longer than one hour.

3. A method according to claim 1 or 2, characterized in that the annealing is applied to a steel bar.

4. Method according to one of claims 1 to 3, characterized in that a rolling bearing steel is used as the bar steel.

5. Method according to claim 4, characterized in that a chromium steel is used as the rolling bearing steel.

6. Method according to claim 5, characterized in that a steel with an alloy of 100Cr6 is used as the chromium steel.

7. Method according to one of claims 1 to 6, characterized in that the semi-finished product is inductively heated in a continuous tempering plant.

8. Method according to one of claims 1 to 7, characterized in that the semi-finished product is heated at a rate of up to 100 K / s.

9. Method according to claim 8, characterized in that the semi-finished product is heated to a target temperature (T1) of up to 880 °C.

10. Method according to claim 9, characterized in that the target temperature (T1) is maintained for a period of up to 300 s. 1 1. Method according to one of claims 1 to 10, characterized in that the semi-finished product is quenched after inductive heating.

12. The method according to claim 11, wherein the semi-finished product is quenched by means of water.

13. A method according to claim 11 or 12, characterized in that the semi-finished product is quenched at a quenching rate of more than 50 K / s.

14. Method according to one of claims 11 to 13, characterized in that the semi-finished product is quenched until it reaches a martensitic state.

15. Method according to one of claims 11 to 14, characterized in that the semi-finished product is heated again after quenching to a second temperature (T2) below the Ac1 temperature.

16. The method according to claim 15, characterized in that the semi-finished product is inductively heated to a second temperature (T2) of 730 °C.

17. Method according to claim 15 or 16, characterized in that the second temperature (T2^) is maintained for a period of up to 500 s.

18. Method according to one of claims 15 to 17, characterized in that the semi-finished product is cooled to room temperature or in still air after the second heating.

19. The method according to claim 18, characterized in that the semi-finished product is heated by at least a second inductive annealing.

20. A method according to claim 18 or 19, characterized in that the semi-finished product is heated after further cooling to a third temperature (T2) below the Ac1 temperature. Semi-finished product produced by a process according to one of claims 1 to 20.

Images