EP2789710B1 - Bismuth containing free-machining steel - Google Patents

Description

The invention relates to a hochzerspanbaren free cutting steel with carbon, silicon, manganese, sulfur, phosphorus, aluminum and iron.

The invention can be used in iron metallurgy and indeed in the production of highly processable free-cutting steel for the production of components in motor vehicle construction.

The patent RU 2437739 C1 discloses a process for producing high sulfur content continuously cast bismuth steels having the following chemical composition by mass fractions: carbon - Max. 0.16; silicon - Max. 0.35; manganese - 1.0 - 1.50; sulfur - 0.08 - 0.35; phosphorus - 0.06 - 0.15; aluminum - Max. 0.03; bismuth - 0.06 - 0.12; Iron and impurities - rest.

• keine Kontrolle / Überwachung der Sauerstoffwirksamkeit vor dem Stahlguss (zwecks Prozessführung und Erzeugung von Nichtmetall-Einschlüssen mit einer vorgegebenen Form sowie Zusammensetzung);
• keine Regelung des Länge-Stärke-Verhältnisses der Partikeln in der Oberflächenschicht und im Mittelpunkt des Querschnitts;
• unzureichende mechanische Schneidbarkeit / Zerspanbarkeit,
• hoher Preis von bismutlegierten Stählen.

The shortcomings of this steel include:

• no control / monitoring of oxygen efficiency before steel casting (for the purpose of litigation and production of non-metal inclusions of a given shape and composition);
• no regulation of the length / strength ratio of the particles in the surface layer and in the center of the cross section;
• insufficient mechanical cuttability / machinability,
• high price of bismutlegierten steels.

in the EP 1449932 A1 is a low-carbon highly processable free-cutting steel with the following composition (by mass) proposed: carbon - 0.02 - 0.15; manganese - 0.05 - 1.8; sulfur - 0.20 - 0.49; oxygen - 0.01 - 0.03; chrome - 0.3 - 2.3 Iron and inevitable impurities - rest; Cr / S value is in the range of 2 to 6.

High cuttability / machinability of free cutting steel is achieved by the ratio Cr-Mn-S. In this steel, chromium is not an inevitable impurity (not an indispensable inclusion), by the introduction of which a globular form of sulphides is achieved. In particular, the minimum chromium content threshold of 0.3% is controlled / monitored, because "reducing the chromium content" is difficult to achieve a sufficient effect of suppressing the elongation (elongation) of the sulfides. can not be achieved sufficient effect of the improved cuttability of steel ". The manganese content as a sulfide-forming element is set in the range of 0.05 to 1.8%, but is preferably in the range of 0.22 to 0.60%, because manganese produces extended sulfides. In an example of the prior art, bismuth occurs, but in combination with lead.

The difference of this steel from that according to our solution is that high cuttability (machinability) is achieved with chromium and not with bismuth. In the prior art, the ratio Cr-S is monitored, not Mn-S. A completely consistent with our solution composition could not be found in this document.

The publication JP 2000 336454 A describes the prototype of the invention which discloses bismuth free steel. This has a perfect high-temperature plasticity and the following composition: carbon - 0.05 - 0.15; manganese - 0.5 - 2.0; sulfur -0.15 - 0.4; phosphorus - 0.01 - 0.1; oxygen - 0.003 - 0.02; bismuth - 0.03 - 0.30; silicon - Max. 0.01; aluminum - Max. 0.00009; Iron and manganese sulphide - inevitable impurities

Steel contains bismuth for distribution at the boundaries of manganese sulfides and in the free state. In this document, the object of increasing the plasticity of bismuthaltigem free cutting steel is achieved in that the stressed steel is rolled while warm, whereby the formation of breakage is excluded. Sulfur partly becomes favorable, separate manganese sulphides, partially bound to iron sulphides, which lead to breakage (red-rupture effects in the steel) during hot rolling. The solution is aimed at exploiting the effect of titanium, boron and nitrogen on the plasticity of the steel when hot.

This technical solution does not suggest such a component ratio Mn-S in the chemical steel composition which would preclude the formation of iron sulfides as well as the hot rolling cracking without introduction of titanium, boron and nitrogen. There is no information on the effect of boron and titanium on the mechanical properties of steel (strength limit, elongation number, hardness). This steel is criticized by the lack of information about the possible influence of oxygen as it changes its share on the geometry and composition of manganese sulfide inclusions for increased turning of the steel.

From the prior art, a lead-containing free cutting steel AS-14 with the following chemical composition, by mass fractions, known: carbon - 0.10 - 0.17; silicon - Max. 0.12; manganese -1.0 - 1.3; sulfur - 0.15 - 0.30; phosphorus - Max. 0.1; lead - 0.15 - 0.30;

This steel is the steel according to the invention its mechanical properties, its composition and its application closest and is considered a prototype.

This steel has the defect that it is dominated by heavily deformed thin-layer inclusions. These cause inferior physical and mechanical properties, a poorer processability of the metals and prevent an increase in machinability. Another deficiency is the toxicity of lead, which is one of the elements of the first hazard class. In the production of lead-containing steels, quite complicated systems are used in steelworks for the extraction of liberated lead vapors. The problem of protection against toxic lead compounds in rolling mills is virtually unresolvable.

It is an object of the invention to increase the cuttability over the entire cross-section and circumference of the rolled steel of free cutting steel while maintaining mechanical properties such as those of lead-containing steels, to improve the environmental situation in the metallurgical industry and to expand the price range of the steels.

The stated object is solved by the features of claim 1.

The bismuth-containing free-cutting steel according to the invention has the following composition by mass: carbon - Max. 0.16; silicon - Max. 0.15; manganese - 1.2 - 1.68; sulfur - 0.2 - 0.4; phosphorus - 0.06 - 0.15; aluminum - Max. 0.01; bismuth - 0.06 - 0.12; total oxygen - 0.003-0.015; Iron and impurities - rest.

The reference designation of this steel according to the invention is AM 14.

A cheap steel with an economical bismuth alloy is proposed with the following proportions by mass: carbon - Max. 0.16; silicon - Max. 0.15; manganese - 1.2 - 1.68; sulfur - 0.2 - 0.4; phosphorus - 0.06 - 0.15; aluminum - Max. 0.01; bismuth - 0.03 - 0.05; total oxygen - 0.003-0.015; Iron and impurities - rest.

The reference designation of this steel according to the invention is AM 12.

The technical problem is solved in that an alloy with sulfur and bismuth is made and evenly distributed ellipsoidal and round sulfide inclusions formed in the metal. The extent of sulfide inclusions depends on the sulfur content. Their morphology depends on the degree of steel deoxidation and oxygen content in the steel as well as on the cooling rate during crystallization. To increase the machinability, the sulfides are best rounded and almost globular and little deformed. They are produced in mild steel with a total oxygen content of 0.0030 - 0.0150%. For this, the oxygen efficiency in the steel is maintained at 20-70 ppm during transfer to the cast steel. The presence of almost globular, low-strain sulfides in the metal is well consistent with the levels of active oxygen and residual aluminum: the higher the oxygen content with smaller residual aluminum content, the more globular sulfides are present in the metal.

The maximum aluminum content of 0.01% is limited by a reduction in the cuttability of the components.

The carbon content of max. 0.16% ensures that the required mechanical characteristics are achieved. If the upper limit of the carbon content is exceeded, the fluidity decreases and the hardness increases. For this reason, it is not possible to use the steel as intended.

The manganese and sulfur contents provide a ratio in the range of 3.4 to 8.0. At this ratio, the red-fracture effects in the steel are less likely. A sulfur content of less than 0.2% leads to a reduction in the acceptable level of machinability.

The lower limit of the phosphorus content of 0.06% increases the steel machinability. If the phosphorus concentration exceeds 0.15%, this has a negative effect on the metal flowability.

The minimal bismuth content of 0.03% in the steel is due to the fact that the machinability inherent in the lead-containing steel is achieved. The maximum bismuth content of 0.12% is, according to experience, selected for optimum casting conditions in a continuous casting plant and the compliance with requirements for the maximum permitted bismuth concentration (HZK) in the air (this HZK value is according to experience 0.5 mg / m ³ ) ,

Fig. 1

eine Aufnahme eines Kleingefüges eines modifizierten Automatenstahls mit einer Korngröße von 8 - 9 in einer der Schmelzen bei 100-facher Vergrößerung, wobei die anhand einer Messleiste eingestellte Länge 400 µm beträgt,

Fig. 2

eine Aufnahme eines Kleingefüges mit körnigem und streifigem Perlit (wobei der streifige Perlit in diesem Verhältnis dominiert) bei einer 500-fachen Vergrößerung, wobei die anhand einer Messleiste eingestellte Länge 90 µm beträgt,

Fig. 3

die Verteilung und die Form der Sulfideinschlüsse im modifizierten Automatenstahl in einer der Schmelzen, wobei die Oberflächenschicht eines Längsschliffs bei 100-facher Vergrößerung wiedergegeben ist,

Fig. 4

die Verteilung und die Form der Sulfideinschlüsse in der Oberflächenschicht eines Längsschliffs des modifizierten Automatenstahls bei 500-facher Vergrößerung,

Fig. 5

die Verteilung und die Form der Sulfideinschlüsse in der Mitte des Längsschliffs bei 100-facher Vergrößerung aus einer Probe des modifizierten Automatenstahls in einer der Schmelzen und

Fig. 6

die Verteilung und die Form der Sulfideinschlüsse in der Mitte des Längsschliffs bei 500-facher Vergrößerung aus einer Probe des modifizierten Automatenstahls in einer der Schmelzen.

The invention will be explained in more detail with reference to the accompanying drawings. Show it:

Fig. 1

a photograph of a small amount of a modified free-cutting steel with a grain size of 8 - 9 in one of the melts at 100x magnification, wherein the set by a measuring bar length is 400 microns,

Fig. 2

a photograph of a small filing with granular and striated pearlite (the striated pearlite dominating in this ratio) at a magnification of 500 times, the length set by means of a measuring strip being 90 μm,

Fig. 3

the distribution and shape of the sulphide inclusions in the modified free-cutting steel in one of the melts, the surface layer of a longitudinal cut being reproduced at 100 times magnification,

Fig. 4

the distribution and shape of the sulfide inclusions in the surface layer of a longitudinal section of the modified free-cutting steel at 500x magnification,

Fig. 5

the distribution and shape of the sulphide inclusions in the middle of the longitudinal cut at 100x magnification from a sample of the modified free-cutting steel in one of the melts and

Fig. 6

the distribution and shape of the sulphide inclusions in the middle of the longitudinal cut at 500x magnification from a sample of the modified free-cutting steel in one of the melts.

The steel grades according to the invention are produced by the Omutninsk Metallurgiewer ZAO in a steel smelting plant. The steel is deoxidized using aluminum at the tapping from the steel melting plant to the pan. The components at an optimum ratio [Mn] / [Si] ≦ 3 for deoxidation are supplied to the bottom portion of a pan. In the ladle furnace, the metal treatment is carried out including argon inflation and lime-clay slag adjustment. After thickening the slag with magnesite powder, powder wire is introduced with a filler elemental sulfur. Subsequently, a wire with a filler bismuth (MnBi) is introduced. The casting takes place in the continuous casting plant according to the "underbody" method. In this case, the continuously cast steel is produced.

The strand is rolled in the hot rolling mill according to procedural instructions and rolling plans by the Omutninsk Metallurgiewer ZAO. Thereafter, the rough rolling stock is formed in a 10-tonne drawing machine into a bright steel finished profile, in circles ranging from 10 to 27 mm and in a hexagon ranging from 14 to 27 mm.

Melting of steel AM 12 and AM 14 with the composition according to the invention was produced. The obtained chemical composition is shown in Table 1 in comparison to the prototype.

The mechanical properties and microstructure of the steels AM 12 and AM 14 were evaluated by the Omutninsk metallurgical plant ZAO in the testing laboratory for comparative tests. The mechanical properties were tested in a 25-tonne tearing machine from QUASAR 250. The Brinell hardness test was performed in a hardness tester type TSch-2M. The mechanical test results for the known and the claimed bright steel are shown in Table 2. From the sample melts, a few lots of profiles were produced in different sizes. The certain dispersion of the strength properties is due to the reduction rate when pulling the profiles of different sizes.

The small steel structure, shape and distribution of sulfide inclusions were investigated in the NEOPHOT 21 microscope. The small structure of steel is ferritic-pearlitic with a predominantly striped pearlite and a grain size of max. No. 5. The grain size was determined on the cross-section of the bright steel profile at 100x magnification according to GOST 5639 ( Fig. 1 ) rated. The ratio of grain perlite to streaky perlite was determined by cross-sectioning at 500x magnification according to GOST 8233 ( Fig. 2 ) rated. Differences in the microstructure of the steel grades AM 12 and AM 14 according to the invention are not present.

Evaluation of the shape of non-metal inclusions revealed the presence of uniformly distributed, singly, slightly deformed, rounded (ellipsoidal) sulfides on roll-and-draw deformed metal, and lack of accumulation of thin-film inclusions that degrade physical-mechanical and process metal properties. The ratio of the length of the sulfide particles to their thickness is 2: 4 in the surface layer ( Fig. 3 . 4 ) and 4: 6 at the center of the cross section ( Fig. 5 . 6 ).

The resulting form of sulfide inclusions ensures a reduction in the adhesion interactions between the material and the tool. Consequently a surface roughness and a wear rate of the cutting tool (the tool life) as achieved with lead-containing steels.

The large-scale tests of the cuttability of rolled products made of free-cutting steel according to the invention were carried out in order to test the stability of the cutting tool, the surface roughness and the chip behavior.

On the basis of functional samples, some companies (OJSC Ulyanovsk Autowerk, OAO Avtodetal-Service, OOO Laguna, St. Petersburg, ZAO Okulovsk furniture fitting factory, etc.) reported positive results of turning the steel AM 12. The stability of the cutting tools was increased by 15-20%. The chip can easily clatter and does not accumulate in the processing area.

OOO Avtopartner, Dimitrovgrad, emphasizes an improvement in the surface quality of the treated workpieces, which has increased by 1 - 2 classes. Following the use of an inspection lot, OOO PROSAM, Ryazan, has reported stable accuracy of high surface quality, testable component dimensions, with the metal having no thread rolling splits.

The chemical composition according to the invention, the deoxidation process, the melting, rolling and setting process enable bright steel products of two steel grades with increased machinability over the entire cross-section and circumference of the rolling stock compared to the lead-containing steels AS 14, while maintaining mechanical properties such as those of the steel AS 14 from the prior art to get in various price ranges, the environmental situation in the metallurgical industry is improved. Table 1 No. stole Chemical composition, % C Mn Si P S Bi al O pb 1 7780-1 0.11 1.45 0,060 0.072 0.247 0,040 - 0.0037 2 4397-1 0.1 1.47 0,002 0,076 0.263 0,030 - 0.0035 ON 12 ≤0,16 1.2 to 1.68 ≤0,15 0.06-0.15 0.2-0.4 0.03-0.05 ≤0,01 0.003-0.015 1 4690-1 0.1 1.5 0.03 0.078 0.260 0.06 - 0.0034 AT THE 14TH ≤0,16 1.2 to 1.68 ≤0,15 0.06-0.15 0.2-0.4 0.06-0.12 ≤0,01 0.003-0.015 Prior art AS14 0.1 to 0.17 1.0-1.3 ≤0,12 ≤0,1 0.15-0.3 - - 0.15-0.3 No. stole Mechanical properties of bright steel Min. Strength limit G _B , MPa Min. Strain number δ,% Hardness HB, max. 1 7780-1 610-620 11 207 2 4397-1 634-639 10-11 197 Proposed steel AM 12 490 10 217 1 4690-1 515-519 12-13 187 Suggested steel AM 14 490 10 217 Prior art AS14 490 10 207

Reference: State of the art:

1. [1.] GOST 1414-75, Gosstandard of Russia, M., 1992, pp. 4-5, 9

Claims (1)

1. Machining steel with high cut by chip removal properties with carbon, silicon, manganese, sulphur, phosphor, aluminium and iron,
   characterised in
   that it additionally comprises bismuth and a defined amount of oxygen, wherein the oxygen efficacy during transfer of the steel towards steel casting is 20 to 70 ppm,
   that the evenly distributed sulfide inclusions have an almost global form and are slightly deformed and
   that the machining steel comprises the following component ratio by mass fraction: Carbon max. 0,16; Silicon max. 0,15; Manganese 1,2 - 1,68 Sulfur 0,2 - 0,4; Phosphor 0,06 - 0,15; Aluminium max. 0,01; Bismuth 0,03-0,05 or 0,06-0,12 Total oxygen content 0,003-0,015; Iron and impurities remainder.

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