Free-machining steel and method for producing same

Abstract

A free-cutting steel with a weight fraction of 0.04 to 0.14% carbon, 0.01 to 0.05% silicon, 0.90 to 1.50% manganese, 0.27 to 0.40% sulfur, 0.001 to 0.14% lead, 0.001 to 0.05% bismuth, up to 0.11% phosphorus, 0.004 to 0.010% boron, 0.008 to 0.015% nitrogen, 0.0005 to 0.004% calcium, 0.001 to 0.005% aluminum, 0.005 to 0.010% oxygen, and a maximum of up to 0.8% process-related impurities, has a nitrogen / boiler ratio of < 2.0, with additional oxide inclusions present with the following relative weight fractions: 0 to 25% B2O3, 15 to 60% Al2O3, 25 to 55% MnO, 5 to 35% CaO and less than 10% SiO2. To produce it, a steel melt with the appropriate composition is subjected to a metallurgical melt treatment with a slag, potassium is added, and the melt is cooled while avoiding reoxidation reactions.

Description

Technical field

[0001] The invention relates to a free-cutting steel according to the preamble of claim 1 and to a method for its production. State of the art

[0002] Free-cutting steels are steels containing at least 0.15 wt% sulfur and are particularly suitable for machining on automatic machines due to their good machinability. Standard free-cutting steels are described in the ISO 683-4 standard.

[0003] Free-cutting steels (with approximately 0.1 wt.% carbon) are typically alloyed with at least 0.15 wt.% lead to improve their machinability. The heavy metal lead has a low melting point and therefore acts as a liquid metal embrittlement at low temperatures. During the machining process, the liquid lead also forms a space between the tool and the workpiece, thus reducing friction. This reduces cutting forces, extends tool life, and improves the surface finish of the component.

[0004] Since lead is classified as a reproductively toxic heavy metal, its use should be avoided whenever possible. For this reason, there are political efforts to limit or completely ban lead as an alloying element in steel. Since the 1980s, lead has sometimes been replaced by bismuth (e.g., US patent 4,247,326). Like lead, bismuth is a heavy metal with a low melting point. Bismuth-alloyed free-cutting steels therefore behave similarly to lead-alloyed free-cutting steels. Significantly less is known about bismuth's toxicity than about lead. However, due to its similarity to lead, it must be assumed that alloying with bismuth is not an environmentally friendly solution. Furthermore, bismuth-alloyed steels are more expensive to produce and thus considerably less economical than lead-alloyed steels.

[0005] Numerous earlier patents demonstrated that the addition of boron, in combination with other precipitates in steel, can lead to a significant improvement in machinability. However, compared to lead-alloyed steels, limitations regarding workability, performance characteristics, and cost-effectiveness remain.

[0006] As early as 1941, patent US2388214 described a product containing 0.004 to 0.02 wt% boron. The addition of boron allowed for a reduction in sulfur content (to improve hot ductility) without impairing machinability. There is no evidence regarding machinability compared to leaded steels.

[0007] Patent US4061494, published in 1977, described a free-cutting steel in which graphite serves as a nucleation site for the graphitization of the carbon contained in the steel. On the one hand, the graphite precipitates embrittle the soft ferrite; on the other hand, graphite acts as a solid lubricant. The machinability of graphite steels is comparable to that of leaded free-cutting steels. However, the graphite precipitates must be generated through a lengthy and expensive heat treatment, which makes these steels commercially unattractive. A corresponding rolled product, named POSGRAM, was introduced by the Korean steel manufacturer POSCO in 2020. Due to the alloying concept, particularly the significantly higher carbon content, the performance characteristics of graphite and free-cutting steels differ, meaning that these products can only be used to a very limited extent as replacements for leaded steels.

[0008] Also in 1977, East German patent number 129664 described a product in which low-melting-point boron oxides were intended to improve machinability. To suppress boron nitride formation, the nitrogen content in this steel is limited to 0.006 wt.%. The boron content is between 0.005 and 0.015 wt.%. For the desired boron oxides to form, at least 150 ppm oxygen must be present in the steel (report on research project 7210-MA / 113 of the Commission of the European Communities from 1991). Maintaining the low nitrogen and high oxygen contents requires a correspondingly high production effort. The machinability of a lead-alloyed steel could not be achieved.

[0009] Patent WO2004050932 from Nippon Steel, dated 2003, describes a free-cutting steel with 5 to 500 ppm boron for the formation of boron nitrides. To achieve good surface quality on the machined component, the Mn / S ratio must simultaneously be between 1.2 and 2.8. However, according to EUR Report 23890 concerning the study "PACROLP: the prediction and avoidance of cracking in long product hot rolling," published in 2009, the Mn / S ratio in free-cutting steels should be as high as possible (preferably >4.0) to avoid the formation of low-melting-point iron sulfide and the resulting hot cracking. At lower Mn / S ratios, the forming temperature should be increased accordingly. With conventional rolling technology, significant failures due to insufficient surface quality must be expected at a Mn / S ratio below 3.0.

[0010] At the 17th International Congress of Mechanical Engineering in Sao Paulo in 2003, Amorim et al. presented a new free-cutting steel containing 0.05 wt.% bismuth and an unspecified amount of boron. A comparison with a steel containing 0.1 wt.% bismuth showed that the boron-alloyed steel resulted in significantly improved tool life with carbide tools at cutting speeds of around 500 m / min and with high-speed steel tools at cutting speeds of 150–200 m / min.

[0011] Hexagonal boron nitride behaves similarly to graphite as a solid lubricant. Boron nitrides adhere to the primary grain boundaries of the steel and also lead to embrittlement. Significant advantages in machinability have been demonstrated in aluminum-alloyed steels (not free-cutting steels) with low sulfur contents. Patent RU2696798 from 2018 describes a corresponding steel with 0.005 to 0.009 wt% boron. According to Tanake (Int. Journal of Machine Tools & Manufacture 47, 2007), the positive effect of boron is lost if the aluminum content is too low (as in free-cutting steel).

[0012] A later patent application by Nippon Steel & Sumitomo Metal Corp., EP3309272, describes a free-cutting steel with unusually high sulfur contents (0.35 to 0.60 wt.%). Preferred properties are expected at high oxygen contents (80 to 250 ppm) and correspondingly low aluminum (<0.003 wt.%) and silicon contents (<0.01 wt.%). The resulting high oxygen content serves to stabilize the manganese sulfides (note: formation of MnO-MnS precipitates). This steel can also contain up to 200 ppm boron. According to the description, the upper limit for boron addition should be chosen to prevent the formation of boron oxide, as this negatively affects machinability. The very high sulfur contents of this steel are detrimental to its production (hot brittleness) and its service properties (high proportion of mechanical weak points).

[0013] Previous research indicates that the addition of boron via the precipitation of boron nitride and / or in combination with existing precipitates such as manganese sulfides, bismuth particles, or oxides has the potential to achieve significant improvements in machinability. The interactions of the various precipitates are poorly understood, and although many good steel concepts are available today, lead-alloyed steels remain essential in terms of machinability and cost-effectiveness. There is still a pressing need for free-cutting steels with improved machinability and limited lead or bismuth content. Description of the invention

[0014] The object of the invention is to provide a free-cutting steel that avoids, in particular, the aforementioned disadvantages regarding manufacturability, environmental impact, cost-effectiveness, usability, or machinability. A further object of the invention is to provide a method for producing such a free-cutting steel.

[0015] These problems are solved by the free-cutting steel defined in claim 1 and the manufacturing process defined in claim 6.

[0016] Preferred embodiments are defined in the dependent claims and explained further in the text below.

[0017] The following content figures in percent (%) or in parts per million ("ppm") refer to weight percentages unless explicitly stated otherwise.

[0018] The free-cutting steel according to the invention has a weight fraction of 0.04 to 0.14% carbon, 0.01 to 0.05% silicon, 0.90 to 1.50% manganese, 0.27 to 0.40% sulfur, 0.001 to 0.14% lead, 0.001 to 0.05% bismuth, up to 0.11% phosphorus, 0.004 to 0.010% boron, 0.008 to 0.015% nitrogen, 0.0005 to 0.004% calcium, 0.001 to 0.005% aluminum, 0.005 to 0.010% oxygen, and a maximum of up to 0.8% process-related impurities, characterized in that the N / B ratio is < 2.0 and that additionally oxide inclusions with the following relative weight fractions are present: 0 to 25% B₂O₃ , 15 to 60% Al2O3, 25 to 55% MnO, 5 to 35% CaO and less than 10% SiO 2 .

[0019] The process-related impurities include in particular chromium (Cr), nickel (Ni), molybdenum (Mo), copper (Cu) and tin (Sn).

[0020] In an advantageous embodiment (claim 2), the phosphorus content is at least 0.04 wt.%

[0021] In a further advantageous embodiment (claim 3), the lead content as well as the bismuth content are below 0.02 wt.%.

[0022] In a further advantageous embodiment (claim 4) the oxide inclusions have the following relative weight fractions: 2 to 25% B 2 O 3 , 15 to 60% Al2O3 , 25 to 55% MnO , 5 to 35% CaO and less than 10% SiO 2 .

[0023] The inventive method for producing the inventive free-cutting steel comprises: Production of a liquid steel melt with a weight fraction of 0.04 to 0.14% carbon, 0.01 to 0.05% silicon, 0.90 to 1.50% manganese, 0.27 to 0.40% sulfur, 0.001 to 0.14% lead, 0.001 to 0.05% bismuth, up to 0.11% phosphorus, 0.004 to 0.010% boron, 0.008 to 0.015% nitrogen, 0.0005 to 0.004% calcium, 0.001 to 0.003% aluminum, 0.005 to 0.010% oxygen, and a maximum of up to 0.8% process-related impurities result in an oxygen activity of 40 to 70 ppm at 1600°C; the oxygen activity is measured during the process; it is essentially a function of the elements C, Ca, Al, Si, Mn, B and is established in thermodynamic equilibrium; a metallurgical melt treatment is carried out with a slag whose composition has the following relative weight fractions: 30 to 40% CaO, 5 to 20% SiO₂, 25 to 35% Al₂O₃, and 10 to 25% MnO, with the aim of adjusting the aluminum content in the melt and conditioning any existing oxide inclusions; calcium is added to control the melting point and / or the hardness of the oxides; the melt is cooled while avoiding reoxidation reactions.

[0024] It was surprisingly found that the steel composition according to the invention is more machinable than standard free-cutting steels according to ISO683-4 with a boron content <40ppm and, in particular, is also more machinable in drilling operations than free-cutting steel with a lead content of 0.15 - 0.30 wt.%.

[0025] Without being bound to any specific theory, it is assumed that the surprisingly good machinability is due to a beneficial, but previously unknown, interaction between boron and Al₂O₃-containing oxides. The significantly improved machinability manifests itself in considerably longer tool life.

[0026] Limiting the carbon content to 0.14 wt.% ensures a predominantly ferritic microstructure. This microstructure is easily formable and results in low cutting forces during chip formation. This, in turn, ensures low energy consumption and long tool life.

[0027] The lower limit of 0.04 wt% carbon results from the steelmaking process. The melting of the steel with electrical energy and the use of graphite electrodes mean that the carbon content in the steel bath cannot be kept below 0.04 wt%.

[0028] A maximum silicon content of 0.05 wt.% ensures that no hard and abrasive SiO₂ inclusions form. Silicon competes with boron for oxide formation. A high silicon content prevents the formation of the beneficial boron-modified oxides. The lower limit of silicon to 0.01 wt.% ensures that the total oxygen content in the steel remains below 100 ppm and that excessive MnO does not form in the oxides.

[0029] Manganese combines with sulfur to form manganese sulfides. These improve machinability. A manganese content of 0.9 wt.% should not be undercut, otherwise too few manganese sulfides will form and machinability will deteriorate. Furthermore, a low Mn / S ratio could lead to the formation of a low-melting-point iron sulfide, which significantly impairs the castability and rollability of the steel.

[0030] Manganese also contributes to the strengthening of the solid solution in ferrite. Exceeding the maximum manganese content of 1.50 wt.% results in such a hardening of the steel that it becomes noticeably more difficult to machine.

[0031] The embrittlement effect of phosphorus negatively impacts the castability and rollability of the steel. Therefore, the phosphorus content is limited to a maximum of 0.11 wt.%. Phosphorus embrittles the ferrite and improves machinability. A preferred embodiment of the steel according to the invention therefore contains at least 0.04 wt.% phosphorus.

[0032] Sulfur forms manganese sulfides, which ensure good machinability. A minimum content of 0.27% by weight must be maintained to achieve the desired machinability. The manganese sulfides weaken the steel structure and impair its performance properties. If the Mn / S ratio is too low, red brittleness develops during rolling, and the desired surface quality is no longer achieved. For these reasons, the sulfur content is limited to 0.4% by weight.

[0033] Lead has an in-situ lubricating and simultaneously embrittlement effect in machining. Productivity in the machining process can be significantly increased by adding lead. Free-cutting steels according to ISO 683-4 contain between 0.15 and 0.35 wt.% lead. To reduce lead as a toxic element in steel, the maximum lead content should therefore be 0.14 wt.%, preferably even a maximum of 0.02 wt.%.

[0034] Bismuth acts like lead. The addition of bismuth is not regulated in ISO 683-4. Typical levels range between 0.05 and 0.1 wt.%. To reduce the toxic element bismuth, a maximum bismuth content of 0.05 wt.% is therefore required. A preferred formulation contains no more than 0.02 wt.%.

[0035] Since aluminum is a strong oxidizing element and also forms nitrides, it influences the composition of the oxides and competes with boron for nitride formation. For this reason, the aluminum content must be carefully controlled within a narrow range. An aluminum content below the minimum of 0.001 wt% is not achievable with the current state of the art in the manufacturing process described here. An aluminum content above the maximum of 0.005 wt% leads to the undesirable formation of crystalline and abrasive Al₂O₃.

[0036] Boron forms a hexagonal boron nitride with nitrogen, which, like graphite, is a solid lubricant. Boron also acts as a deoxidizer and forms complex CaO-Al₂O₃-SiO₂-MnO-B₂O₃ compounds, or it precipitates locally as boron oxide on existing oxides.

[0037] Boron nitrides are preferentially located at the former austenite grain boundaries, where they have an embrittlement effect. At a boron content above 0.010 wt.%, this leads to a pronounced ductility minimum in the steel at approximately 1100°C. This results in serious surface cracks during both continuous billet casting and the subsequent hot rolling process, making the production of conformal rolled steel impossible.

[0038] No significant improvement in machinability was observed at a boron content below 0.004 wt.%.

[0039] Nitrogen forms boron nitrides and leads to embrittlement of the ferrite. Both of these factors improve the machinability of the steel. At a nitrogen content below 0.008 wt%, the nitrogen is completely consumed by boron nitride formation. As a result, the ferrite is no longer sufficiently work-hardened, leading to increased adhesive wear at low machining speeds (below 100 m / min). At a nitrogen content above 0.015 wt%, too many boron nitrides are formed, and the steel can no longer be cast and rolled as desired.

[0040] Free-cutting steels are often produced with an oxygen content above 0.01 wt.%. The high oxygen stabilizes the manganese sulfides and reduces anisotropy caused by elongated manganese sulfides. However, high oxygen contents lead to an unfavorable MnO / Al₂O₃ ratio, which negatively impacts tool life. For this reason, the upper limit of the oxygen content should be set at 0.01 wt.%.

[0041] Free-cutting steels are silicon-manganese deoxidized steels. Under thermodynamic equilibrium, the oxygen content in the molten steel reaches a stable equilibrium. Due to the steel composition, the achievable lower limit for the oxygen content is 0.005 wt.%. Ways to implement the invention

[0042] Exemplary embodiments of the invention are described in more detail below with reference to the drawing, which shows: Fig. 1 a graphical representation of the determined tool life (in minutes) as a function of the bore content of various steels; and Fig. 2 photos of chips after external longitudinal turning for a) a Si-alloyed steel 11SMn30+Si, b) a boron- and Si-alloyed steel 11SMn30+Si+Boron and c) a boron-alloyed steel according to the invention 11SMn30+Boron. Fig. 3 a comparison of a) a steel according to the invention (graphic left) with b) a non-inventive lead-alloyed, Ca-treated steel (graphic right), wherein the cutting speed is plotted as a function of the feed rate.

[0043] In exemplary embodiments, molten steel was cast and subsequently rolled into wire rod. The molten steel was produced using the electric arc furnace process with secondary metallurgical treatment in a ladle and subsequent casting into 152 x 152 mm billets in a continuous casting plant. The billets were then reheated in a walking beam furnace to 1,200 to 1,250°C and subsequently rolled into wire rod with diameters of 26 to 30 mm. The coils were cooled after rolling in a fan of moving air.

[0044] In bright steel production, the wires were blasted to remove the iron oxide layer and straightened. Afterwards, the bars were drawn to their final dimensions through a drawing die and then straightened again.

[0045] Turning tests to determine tool life were performed on a DMG Mori CTX400 lathe at a cutting speed of 200 m / min. The bars were fed into the machine via a bar feeder. The tests were conducted without coolant using a coated indexable insert CNMG120308 QM235. The depth of cut was 1.25 mm at a feed rate of 0.125 mm / rev. The test was carried out until the tool life criterion of 0.3 mm flank wear was reached.

[0046] Table 1 shows the service life results of 12 test melts and three reference batches, one without lead (11SMn30) and two with lead (11SMnPb30 and 11SMnPb37). The lighter elements carbon, nitrogen, and oxygen were determined chromatographically (using a LECO instrument). The highlighted areas show the deviations from the inventive design of the steel product. The service lives of the non-inventive designs range from 35 to 65 minutes (for the lead-alloyed variant). The service lives of the inventive variants vary between 76 and 257 minutes, with a mean value of 122 minutes.

[0047] The composition of non-metallic inclusions in the steel was determined using EDX (energy-dispersive X-ray spectrometry) in a scanning electron microscope. The measurement area was at least 40 mm² (not exactly the same for every test variant). A grid resolution of 0.391 µm was used for scanning. Isolated manganese sulfides were filtered out using a neutral density filter. Only precipitates with an equivalent radius between 2 and 5 µm and measurements with C < 10% and O > 10% were considered in the analysis. Larger inclusions are rare in these steels and may be due to isolated events during casting. With smaller inclusions, the larger measurement spot results in predominantly ambient measurements, and the measurement amplitudes of the corresponding spectral peaks become small (or the measurement errors become large). Because boron precipitates are of particular interest, the lighter elements B and O were analyzed despite the known measurement inaccuracies.The derived proportions of B₂O₃ should therefore only be considered indicative. In fact, B₂O₃-containing inclusions are always found when evaluating the raw data (without applying filters). The results listed in Table 2 were normalized to 100% in the B₂O₃-SiO₂-MnO-CaO-Al₂O₃ system. Deviations from the inventive design are highlighted.

[0048] Figure 1 This shows that the addition of boron alone (not according to the invention) does not lead to an improvement in service life. The determined service life remains practically constant at approximately 50 minutes. With increasing boron alloying, the service life also increases. The preferred boron content is between 60 and 80 ppm.

[0049] To indirectly detect the presence of boron oxide, a boron-alloyed batch was additionally alloyed with silicon (0.198 wt%). Silicon has a higher affinity for oxygen than boron and prevents the formation of boron oxide. Three steel variants were compared: 11SMn30 + silicon, 11SMn30 + silicon + boron, and 11SMn30 + boron (according to the invention). The steel analyses are comparable, with the exception of silicon and boron. The chip breakage behavior in the turning test (external longitudinal turning at 200 m / min, 1.5 mm depth of cut) was investigated at feed rates of 0.1 mm / rev and 0.125 mm / rev, respectively. Figures 2a-cThe images show photos of chips. The chip fracture behavior at f=0.125mm / rev deteriorates for the silicon-alloyed variant due to the addition of boron. The boron exclusively forms boron nitride, and the nitrogen is no longer available in sufficient quantity for the work hardening (and embrittlement) of the ferrite. In the steel analysis according to the invention, with a standard-compliant (ISO 683-4), low silicon content, an optimal chip shape and optimal chip fracture behavior are observed after the addition of boron.

[0050] The inventive variant V12 was used in a drilling test on a MAZAK INTEGREX i300 multi-function machine. BLAZER VASCO 7000 was used for external lubrication of the drill bit. The 4 mm diameter HSS drill bit was a Tivoly M2 type N drill bit. The hole length was set to four times the diameter.

[0051] To determine the optimal working window (OOZ), tool life tests were conducted for various Vc-f pairs. This allowed the determination of the upper limits of the OOZ and the maximum material removal rate. Each tool life test was performed until tool breakage or until a specific number of holes (1,140 holes) had been drilled.

[0052] The optimal working window (OOZ) was defined as follows: If 1,140 holes (formerly 1,125) are achieved without the drill bit breaking, the tested pair "Vc - f" is within the OOZ ("good"). If the drill bit breaks after a number of holes that is less than half the number of holes to be achieved, the pair "Vc - f" is declared outside the OOZ ("bad"). If the drill bit breaks after a number of holes that is between the number of holes to be achieved and half that number, the pair "Vc - f" is assigned to the intermediate zone ("transition zone").

[0053] The Figures 3a-b The results for the non-inventive lead-alloyed steel (11SMnPb30) and the inventive variant V12 are shown. The optimal working range is similar for both steels. The maximum material removal rate (volume removed per minute) achievable under good conditions is 20 cm³ / min. With the non-lead-alloyed steel 11SMn30, only 12 cm³ / min could be achieved. The corresponding productivity of 11SMnPb30 and the inventive steel is therefore 67% higher.

Claims

1. A free-cutting steel with a weight fraction of 0.04 to 0.14% carbon, 0.01 to 0.05% silicon, 0.90 to 1.50% manganese, 0.27 to 0.40% sulfur, 0.001 to 0.14% lead, 0.001 to 0.05% bismuth, up to 0.11% phosphorus, 0.004 to 0.010% boron, 0.008 to 0.015% nitrogen, 0.0005 to 0.004% calcium, 0.001 to 0.005% aluminum, 0.005 to 0.010% oxygen, and a maximum of up to 0.8% process-related impurities. characterized by the fact that the N / B ratio is < 2.0 and that additionally oxide inclusions are present with the following relative weight fractions: 0 to 25% B2O3, 15 to 60% Al2O3, 25 to 55% MnO, 5 to 35% CaO and less than 10% SiO2 2. The free-cutting steel according to claim 1, wherein the phosphorus content is at least 0.04 wt.%.

3. The free-cutting steel according to claim 1 or 2, wherein both the lead content and the bismuth content are below 0.02 wt.%.

4. The free-cutting steel according to claim 1, 2 or 3, characterized by the fact thatThe oxide inclusions have the following relative weight fractions: 2 to 25% B2O3, 15 to 60% Al2O3, 25 to 55% MnO, 5 to 35% CaO and less than 10% SiO2.

5. A method for producing free-cutting steel according to claim 1, wherein: a) in a liquid steel melt with a weight fraction of 0.04 to 0.14% carbon, 0.01 to 0.05% silicon, 0.90 to 1.50% manganese, 0.27 to 0.40% sulfur, 0.001 to 0.14% lead, 0.001 to 0.05% bismuth, up to 0.11% phosphorus, 0.004 to 0.010% boron, 0.008 to 0.015% nitrogen, 0.005 to 0.004% calcium, 0.001 to 0.005% aluminum, 0.005 to 0.010% oxygen, and a maximum of up to 0.8% process-related impurities, such that at 1600°C a a) adjusts oxygen activity to 40 to 70 ppm; b) performs a metallurgical melt treatment with a slag whose composition has the following relative weight fractions: 30 to 40% CaO, 5 to 20% SiO2, 25 to 35% Al2O3 and 10 to 25% MnO, to adjust the aluminium content in the melt and to condition any existing oxide inclusions; c) adds calcium; d) cools the melt while avoiding reoxidation reactions.

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