EP0538158A1 - Low alloy, resulfurized free-cutting steel - Google Patents

Abstract

Steels of particular compositions which, because of their sulphur contents and of the composition of the oxidised inclusions, are particularly suitable for screw-cutting and machining at high speed using carbide or ceramic tools. The steels contain, in weight %: C - 0.05 to 0.45 % Mn - 0.80 to 1.65 % Si - 0.10 to 0.35 % P - max. 0.030 % S - 0.050 to 0.12 % and the compositions of their oxidised inclusions are, in weight %: Al2O3 - 15 to 50 % SiO2 - 20 to 60 % CaO + MnO - 15 to 50 %. Despite the presence of sulphur, these steels retain good mechanical characteristics.

Description

The present invention relates to steels weakly alloyed with carbon-manganese and relates to steels of particular compositions which, in particular because of their sulfur content and of the composition of their oxidized inclusions, are particularly suitable for high speed machining in in particular by carbide or ceramic tools and by cutting at high cutting speed, in particular from tubes produced in said steels.

Steels are currently known whose machinability characteristics are improved by the presence of sulfur in a content range from 0.020 to 0.040% by weight. However, these steels have limited machining performance which does not allow high cutting speeds to be achieved and / or the use of carbide or ceramic cutting tools under good conditions. On the other hand, these steels with improved machinability generally keep mechanical characteristics, in particular in traction, close to those corresponding to their basic composition.

There are also known steels for bar turning semi-calmed by silicon additions containing about 0.3% sulfur by weight. These steels can be machined at high cutting speed with carbide or ceramic cutting tools.

In some cases, even, the search for the highest performance requires the combined addition of sulfur, tellurium (or selenium) and / or lead or bismuth.

These steels generally have a large amount of inclusions of oxides and sulfides and a high oxygen content greater than 0.0100% by weight.

These steels, the essential characteristic of which, if not exclusive, is that they have properties that are as high as possible in bar turning, have, moreover, very poor resilience and it is impossible to assemble them by arc welding. We can also highlight the toxicity problems associated with handling, especially at the the elaboration of the liquid metal of the Tellurium, Selenium, Lead, Bismuth elements as well as during the recycling of scrap and chips

In addition, steels with 0.30% by weight of sulfur have low mechanical tensile properties, close to that of A37 steel according to AFNOR standard. Nuances for heat treatment have been standardized, but their bar turning performance is therefore weaker.

The aim of the present invention is to propose a family of carbon-manganese steels which can be used in particular for mechanical construction having an excellent turning ability and giving excellent performance at high speed machining with carbide or ceramic tools. , performances superior to those obtained with steels with improved machinability cited above while retaining good mechanical characteristics and not having the drawbacks of free-cutting steels at 0.30% by weight of sulfur mentioned above.

The object of the invention is in particular to create a family of clean free-cutting steels, not requiring the use of toxic elements, covering a wide range of mechanical characteristics and having much better impact toughness characteristics than those free-cutting steel at 0.30% by weight of sulfur.

Carbone

- 0,05 à 0,45 %

Manganèse

- 0,80 à 1,65 %

Silicium

- 0,10 à 0,35 %

Phosphore

- inférieur à 0,030 %

ainsi que les impuretés habituelles.The carbon-manganese steels suitable for bar turning of the family of the invention have the following composition expressed in weight percentages:

Carbon

- 0.05 to 0.45%

Manganese

- 0.80 to 1.65%

Silicon

- 0.10 to 0.35%

Phosphorus

- less than 0.030%

as well as the usual impurities.

Soufre

- 0,050 à 0,12 %

Calcium

- 0,001 à 0,010 %

l'aluminium étant entièrement sous forme oxydée, et les inclusions non métalliques d'oxydes comprennent en pourcentage pondéraux :

Al ₂ O₃

15 à 50 %

Si O₂

20 à 60 %

CaO + MnO

15 à 50 %

la teneur totale en oxygène étant comprise entre 0,0015 et 0,0080 %.They also contain:

Sulfur

- 0.050 to 0.12%

Calcium

- 0.001 to 0.010%

aluminum being entirely in oxidized form, and the non-metallic inclusions of oxides comprise in percentage by weight:

Al ₂ WHERE

15 to 50%

If where

20 to 60%

CaO + MnO

15 to 50%

the total oxygen content being between 0.0015 and 0.0080%.

Preferably the non-metallic inclusions are located on the diagram of representation of the system Al ₂ O₃ - Si O₂ - MnO / CaO in the domain of Anorthite or Gehlenite. Such a representation diagram of the system Al ₂ O₃ - Si O₂ - MnO / CaO can be found for example in "Phase diagrams for ceramists" Edited and Published by the American Ceramic Society 1964 figure 630 page 219. MnO and CaO are oxides whose role is estimated to be similar. Insofar as the percentage of MnO is low (less than 10 '/.) We can therefore cumulate CaO + MnO to refer to the diagram of the system Al ₂ O₃ - Si O₂ - CaO where only CaO is represented and take the value of CaO + MnO for a value of CaO.

Advantageously for the implementation of steel by welding by an electric arc welding process with filler metal, the carbon content is limited to 0.22% by weight and the sum carbon content plus content in manganese / 6 at 0.48% by weight, ie C% + $$\frac{\text{Mn\%}}{\text{6}}\text{≦ 0.48\%.}$$

Advantageously also the sulfur content is between 0.080% and 0.12% by weight.

The composition described above makes it possible, while obtaining good mechanical properties under tension, to obtain a type of favorable inclusion of oxides and sulphides, distributed homogeneously in the metal and in a limited and controlled quantity. This set of characteristics makes these steels suitable for bar turning in excellent conditions, this despite the relatively low sulfur content, in particular for bar turning using carbide or ceramic tools.

It is particularly advantageous for manufacturing steel to use the rotary continuous casting process, as is known.

This process is for example described in the article of the CIT metallurgical review, February 1981 pages 119-135.

Such a process makes it possible to ensure the homogeneity of the metal, both in terms of the chemical composition and the distribution of the inclusions, over the entire length of casting.

The production of steel which is then continuously cast, which is advantageously made in a ladle after fusion in an electric arc furnace, will be carried out with care so as to always keep the metal homogeneous and protected from oxidation, the metal being, in its final phase treated with Calcium, for example by the introduction into the pocket of liquid metal of a cored wire as described in European patent n ° 87 420052.0 based on silico-calcium.

It is particularly advantageous to use the steel according to the invention in the form of a tube, for example of drawn tubes, this reducing the turning times by eliminating the drilling operation of the internal hole.

By using a tube, one is also certain to avoid the central segregations which could be found at the heart of the bars, segregations which, as already explained above, are very reduced if the rotary continuous casting process is used.

In this way the use of tubes instead of bars is an additional guarantee for the product intended for bar turning.

By way of non-limiting example, the results of a steel according to the invention have been compared with pre-existing standardized steels.

The compositions are as follows in percentages by weight:
- Steel A according to:
TU52b (AFNOR standard NFA 49310) VS : 0.178% Yes : 0.265% Mn : 1.270% S : 0.006% P : 0.014% Al : 0.024% It : 0.0015% 0 total : 0.0022%

The oxide inclusions consist of calcium aluminates Al₂ O₃ - CaO or spinels Al₂ O₃ - Mgo associated with mixed sulfides (Ca, Mn) S. There are also isolated inclusions of manganese sulfides MnS in small quantities.
- Steel B (steel version with improved machinability of steel A).
according to
TU 52 b q2 (AFNOR standard NFA 49310) VS : 0.180% Yes : 0.183% Mn : 1.340% S : 0.028% P : 0.016% Al : 0.022% It : 0.0016% 0 total : 0.0022%

Here the oxide inclusions consist of Al₂O₃ - CaO calcium aluminates or Al₂O₃ - MgO spinels associated with mixed sulfides (Ca, Mn) S. Besides these inclusions, there are numerous inclusions of manganese sulfide.

Steel C according to the invention. VS : 0.172% Yes : 0.191% Mn : 1.430% P : 0.018% S : 0.090% Al : ≦ 0.005% It : 0.0014% 0 total : 0.0026%
having inclusions of oxides the average composition of which is as follows in percentages by weight: Al ₂ WHERE : 34% If where : 30 % CaO : 36%

This composition is located in the Gehlenite domain of the ternary diagram Al₂ O₃ - Si O₂ - CaO cited above.

In this steel C the sulfur is essentially combined in the form of MnS sulphides.

These three steels were subjected to a turning test on a drawn-out tube with a carbide tool, referenced SAFETY TPMR 16 n ° 030476S4 from the company SAFETY, uncoated and without lubricant.

• la durée pour créer une usure de 0,4 mm de l'arête de l'outil à une vitesse de coupe de 180 m/min (durée de vie)
  et
• la vitesse de coupe en m/min donnant une usure de 0,4 mm de l'arête de l'outil en 20 minutes (VB 20-0,4).

We compared:

• time to create 0.4 mm wear on the tool edge at a cutting speed of 180 m / min (lifetime)
  and
• the cutting speed in m / min giving a wear of 0.4 mm of the tool edge in 20 minutes (VB 20-0.4).

The results are as follows:

It can be seen that the steel according to the invention (steel C) presents results far superior to those obtained with the existing improved machinability steel (steel B) and without common measure with the steel not intended for machining (steel AT).

We also compared by a productivity test with carbide tools with identical criteria a free-cutting steel at 0.30% by weight of sulfur with the steel C of the invention.

In 6 hours, almost identical production was found for the two steels, this despite the differences in unfavorable mechanical characteristics for the steel C, steel of the invention:
Steel at 0.30% sulfur weight according to NFA 35561 (grade S300). R ρ 0.2 539 MPa Rm 558 MPa A% 5.65 √S 11.4%

Steel C of the invention R ρ 0.2 704 MPa Rm 765 MPa A% 5.65 √S 8.4%

This clearly shows that the steel according to the invention is particularly suitable for high-speed turning with carbide or ceramic tools.

The steels according to the invention therefore have the advantage of having much better machinability properties than those of steels with improved machinability and of the same order of magnitude as those of free-cutting steels with 0.30% by weight of sulfur without having , especially in terms of mechanical characteristics and implementation of the drawbacks of these steels at 0.30% by weight of sulfur.

Claims (6)

1) Carbon-manganese steel suitable for free cutting at high cutting speed, characterized in that it contains in weight percentages, 0.05 to 0.45% of carbon, 0.80 to 1.65% of manganese, 0 , 10 to 0.35% of silicon, at most 0.030% of phosphorus and the usual impurities as well as 0.050 to 0.12% of sulfur and 0.001 to 0.010% of calcium and in which the non-metallic inclusions of oxides comprise in percentages by weight 15 to 50% of Al₂ O₃, 20 to 60% of SiO₂ and 15 to 50% of CaO + MnO, the total oxygen content being between 0.0015 to 0.0080% and the aluminum being entirely in the form oxidized. 2) Steel according to claim 1 characterized in that the non-metallic inclusions are located on the diagram of the system Al₂ O système - Si O₂ - CaO / MnO in the field of Anorthite or Gehlenite, the CaO + MnO content being treated as CaO. 3) Steel according to claim 1 or 2 characterized in that the sulfur content is 0.080 to 0.12%. 4) Steel according to one of claims 1 to 3 characterized in that in order to have good weldability with an electric arc with filler metal, the carbon content is limited to 0.22% and the sum $$\text{C\% +}\frac{\text{Mn\%}}{\text{6}}\text{at 0.48\%.}$$

5) Steel according to any one of claims 1 to 4 characterized in that it is cast by the rotary continuous casting process. 6) Steel tube for free cutting characterized in that its composition is in accordance with any one of claims 1 to 4.