Method for producing a semi-finished product using pearlitic steel and semi-finished product produced by the method

Abstract

A process for producing a semi-finished product from pearlitic steel by annealing is characterized in that the semi-finished product is annealed by inductive annealing on globular cementite. The process times are shortened compared to the conventional process, and the carbides formed are smaller and more finely dispersed compared to conventional treatment.

Description

[0001] 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.

[0002] During annealing, defects such as dislocations are first healed, and stresses in the material are reduced. Then, recrystallization occurs, in which new nuclei form and replace highly strained crystallites. Finally, crystal growth occurs.

[0003] 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 temperatures; the designations Ac1, Ar1, Ac2, Ar2 etc. are also used in the iron-carbon diagram. A stands for "arrêt", i.e. holding point or inflection point, r for "refroidissement", i.e. cooling, and c for "chauffage", i.e. heating.

[0004] 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 also referred to as annealing to spheroidal cementite (GKZ annealing). This makes the material easier to form and machine.

[0005] Annealing improves the machinability of steels with a carbon content of more than 0.4 mass percent.

[0006] 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 (Fe3C) 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 characterize transformation points, referred to below as Ac1, Ar1, Ac2, and Ar2.A stands for “Arrêt” or stopping point, r for “refroidissement” or cooling and c for “chauffage” or heating.

[0007] The transformation points mentioned can be calculated thermodynamically for a specific chemical analysis. This is usually represented in a time-temperature austenitization diagram (ZTA diagram) ( Fig. 1), where time is shown logarithmically in seconds on the x-axis, while 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. A ferritic-pearlitic initial microstructure is traditionally assumed. When an Ac1 temperature 13 is exceeded, the carbides begin to dissolve, and the pearlite transforms into austenite upon further temperature increase. When an AC3 temperature 12 is exceeded, the transformation of the pearlite is complete, and the remaining ferrite transforms into austenite. Only above a "homogeneous austenite" line 11 is a completely transformed and homogeneous austenite present. 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.

[0008] 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.

[0009] GKZ annealing, sometimes also called spheroidal annealing, is usually divided and characterized into three types: 1. Delayed cooling: The steel is heated to temperatures > Ac1 (line Ac1), kept at temperatures between Ac1 and Ac3 and then cooled very slowly. 2. Isothermal treatment: The steel is heated to temperatures just below Ac1 and kept isothermal for long periods and finally cooled. 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.

[0010] In practice, hybrids of the three basic principles are often used, but they all have in common the very long annealing time and the associated high energy consumption. Typical annealing times range between 15 and 50 hours, depending on the chemical composition.

[0011] 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 as Fe3C (cementite) in a ferritic matrix.

[0012] Annealing at the dissolution temperature (Ac1) allows the carbon in the structure to become mobile and strive for the most energetically favorable state with the smallest possible surface area. Diffusion results in the formation of spheroidal cementite, hence the name GKZ (spheroidal cementite annealing). This makes the material easier to form and machine. Tensile strength and hardness decrease, and elongation at break increases. Bearing steels such as 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%.

[0013] 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: In Fig. Figure 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.

[0014] The initial state of pearlitic steels is characterized by the presence of carbon in the steel as intermetallic phases of the Fe3C 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."

[0015] 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.

[0016] 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.

[0017] 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, carbide annealing has been achieved on various steel grades within significantly shorter times. The starting point is still a pearlitic steel, but warm forming around Ac1 introduces deformation energy into the system. This increases the diffusion rate and accelerates the forming process from pearlite to spheroidal cementite.

[0018] 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%.

[0019] GKZ annealing is used for steels with a carbon content of more than 0.8% (hypereutectoid steels). The goal is to spheroidize the cementite (GKZ = annealing to spheroidal cementite), which is achieved by heating above the transformation temperature Ac1 or by means of a pendulum annealing around the transformation point Ac1 in combination with a very long holding phase and slow furnace cooling.

[0020] 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 for the purpose 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, known as lamellar pearlite.

[0021] 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.

[0022] 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.

[0023] The specifications for microstructure characteristics are defined in a steel-iron test sheet in the form of an illustrated standard series (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 impairs subsequent hardening processes.

[0024] 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.

[0025] Currently, annealing of cast iron is carried out in batch-loaded furnaces or in very long continuous furnaces. Annealing times range from several hours to several days.

[0026] In a standard annealing process, for example, the material is heated to 840°C in a batch-loaded furnace at a heating rate of 100K / h for an excitation phase. After a holding phase for thorough heating, the furnace cools in a controlled manner for 24 hours, so that a holding temperature below the Ac1 temperature of 730°C is maintained for just under 24 hours. At this holding temperature, the fragmentation of the pearlite lamellae and ultimately the spheroidal precipitation occur.

[0027] 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.

[0028] In addition to the pearlite content, the achieved carbide sizes are also important parameters for characterizing the resulting microstructure. Longer annealing times lead to a lower pearlite content, with standard annealing resulting in carbide sizes with a diameter of more than 5 µm (or coarser than 2.3 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.

[0029] 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.

[0030] 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.

[0031] The goal is to maintain a defined proportion of residual austenite in the hardening structure, or even eliminate it entirely. Hardening temperatures are therefore typically between 840 °C and 880 °C. For coarse carbide formation, higher temperatures often have to be selected, which can lead to distortion during the hardening process. These distortions, in turn, require 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.

[0032] According to the current state of the art, GKZ annealing of steel with the 100Cr6 alloy is performed in batch-loaded furnaces or very long continuous furnaces. Annealing times range from several ten hours to several days.

[0033] 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, the material is cooled to a temperature just below Ac1 and this temperature is maintained for 24 hours. During this holding phase, the lamellar pearlite decomposes and precipitates again in spherical form. In order for this fragmentation process to occur uniformly at every point and in every intergranular grain, these very long annealing times are required. 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.

[0034] 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.

[0035] 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.

[0036] According to the invention, this object is achieved as stated in claim 1.

[0037] The present invention provides a method for producing a semi-finished product made of pearlitic steel with spherically formed carbides and a degree of indentation of 100%, ie with a pearlite content of 0%, whereby times are shortened and energy consumption is reduced.

[0038] A method is created for inductive GKZ annealing, for example of a rolling bearing steel, for example in the form of bar steel.

[0039] 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.

[0040] Thus, a semi-finished product is annealed to spheroidal cementite by inductive annealing.

[0041] 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).

[0042] 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.

[0043] The semi-finished product is annealed to spheroidal cementite by inductive annealing.

[0044] 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.

[0045] Advantageous further developments emerge from the subclaims and the embodiments, in particular from the representation in the drawings.

[0046] Induction annealing is preferably applied to a steel bar.

[0047] It is advantageous to use a steel bar that is suitable for use in a rolling bearing (rolling bearing steel).

[0048] The use of chromium steel is particularly advantageous. A steel with an alloy of 100Cr6 is preferred.

[0049] It is advantageous for the semi-finished product to be inductively heat-treated in a continuous heat-treating plant.

[0050] Heating rates that allow heating of the semi-finished product at up to 100 K / s are advantageously used.

[0051] In particular, the semi-finished product is advantageously heated to a target temperature of up to 880 °C.

[0052] It is also advantageous if the target temperature is maintained for a period of up to 300 s.

[0053] Preferably, the semi-finished product is quenched after inductive heating, for example with water or cold air.

[0054] It is advantageous to use a quenching speed of more than 50 K / s.

[0055] It is also advantageous to quench the semi-finished product until it reaches a martensitic state.

[0056] Preferably, after quenching, the semi-finished product is heated again to a temperature below the Ac1 temperature.

[0057] A temperature of up to 730 °C is preferably achieved by inductive heating.

[0058] Preferably, this temperature is maintained for a period of up to 500 s.

[0059] It is advantageous to cool the semi-finished product to room temperature after the second heating.

[0060] It is particularly advantageous if the semi-finished product is heated by at least a second inductive annealing.

[0061] It is also advantageous to heat the semi-finished product again to a temperature below the Ac1 temperature after the second annealing.

[0062] 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.

[0063] The invention is described in more detail below using exemplary embodiments. They show: Fig. 4 a continuous tempering plant for tempering steels in a schematic representation, Fig. 5 a temperature curve for an inductive AC annealing as a function of time and Fig. 6a shows a microstructure in a conventionally annealed semi-finished product taken with a scanning electron microscope. Fig. 6b a microstructure in an inductively annealed semi-finished product taken with a scanning electron microscope

[0064] In a first embodiment, ( Fig. 4) a body formed as a bar steel made of a rolling bearing steel with an alloy 100Cr6 is inductively treated by heat in a continuous quenching and tempering plant 40.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] This is followed by 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, and the holding times are between 2 and 15 minutes, followed by cooling in still air to room temperature at less than 10 K / s.

[0069] 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. Example 1:

[0070] A steel with an alloy 100Cr6, i.e., 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: 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.

[0071] 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.

[0072] 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. Example 2:

[0073] A steel 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.

[0074] In Fig. 6a and Fig. Figure 6b shows micrographs obtained with a scanning electron microscope. The sections were metallographically prepared, etched, and imaged using a scanning electron microscope at 10,000x magnification.

[0075] In the presentation Fig. Figure 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.

[0076] In the presentation Fig. Figure 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.

[0077] 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.

[0078] 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 around the finer carbides during machining, rather than cutting through them in the standard process. This has a positive effect on machining efficiency and tool life, despite the comparatively higher hardness.

[0079] 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.

[0080] Pre-material produced in this way is characterized by hardnesses in the range of 240 HB to 300 HB (Brinell hardness). Carbide deposition of 100% is achieved; significantly finer carbide precipitates are produced compared to state-of-the-art annealing processes. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 4 604 145 A [0006, 0017, 0034] RU 2 238 338 C1

[0034]

Claims

[1] Process for producing a semi-finished product from a pearlitic steel by annealing, characterized by that the semi-finished product is annealed to spheroidal cementite by inductive annealing. [2] Method according to claim 1, characterized by that the annealing is applied to a steel bar. [3] Method according to claim 1 or 2, characterized by that a rolling bearing steel is used as bar steel. [4] Method according to claim 3, characterized by that chromium steel is used as rolling bearing steel. [5] Method according to claim 4, characterized by that a steel with an alloy of 100Cr6 is used as chromium steel. [6] Method according to one of claims 1 to 5, characterized by that the semi-finished product is inductively heated in a continuous tempering system. [7] Method according to one of claims 1 to 6, characterized by that the semi-finished product is heated at a rate of up to 100 K / s. [8] Method according to claim 7, characterized by that the semi-finished product is heated to a target temperature (T1) of up to 880 °C. [9] Method according to claim 8, characterized by that the target temperature (T1) is maintained for a period of up to 300s. [10] Method according to one of claims 1 to 9, characterized by that the semi-finished product is quenched after inductive heating. [11] Method according to claim 10, characterized in that the semi-finished product is quenched by means of water. [12] Method according to claim 10 or 11, characterized by that the semi-finished product is quenched at a quenching rate of more than 50 K / s. [13] Method according to one of claims 10 to 12, characterized by that the semi-finished product is quenched until it reaches a martensitic state. [14] Method according to one of claims 10 to 13, characterized bythat the semi-finished product is heated again to a second temperature (T2) below the Ac1 temperature after quenching. [15] Method according to claim 14, characterized by that the semi-finished product is inductively heated to a second temperature (T2) of 730 °C. [16] Method according to claim 14 or 15, characterized by that the second temperature (T2) is maintained for a period of up to 500 s. [17] Method according to one of claims 14 to 16, characterized by that the semi-finished product is cooled to room temperature after the second heating. [18] Method according to claim 17, characterized by that the semi-finished product is heated by at least a second inductive annealing. [19] Method according to claim 17, characterized by that the semi-finished product is heated after further cooling to a third temperature (T2) below the Ac1 temperature. [20] Semi-finished product produced by a process according to any one of claims 1 to 19.

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