EP0493306A1 - Heat treatable steel, containing no graphite - Google Patents

Abstract

The invention relates to a graphite-free, hyper-eutectoid heat-treatable steel with (in % by mass) 0.91 to 1.30% of C 0.20 to 0.80% of Mn up to 0.30% of Si up to 0.01% of Al 0.10 to 0.30% of Cr up to 0.040% of S up to 0.040% of P up to 0.020% of N not more than 0.1% of Ni not more than 0.1% of Mo 0.0015 to 0.1% of Te, the remainder being iron and unavoidable impurities, the tellurium/sulphur ratio being greater than 0.20.

Description

The invention relates to a graphite-free hypereutectoid tempering steel. According to the iron-carbon diagram, steels with a carbon content of more than 0.80% are referred to as hypereutectoid steels. Their structure consists of cementite and pearlite.

Heat treatable hypereutectoid steels with carbon contents from 0.9 to 1.3%, manganese contents from 0.30 to 0.90% and silicon contents from max. 0.4% and sulfur and phosphorus contents up to max. 0.035% is processed in the form of sheet metal, strip or wire both in the hot-rolled state and after cold rolling to a large extent by cold forming such as bending, folding, straightening, winding, punching and deep drawing. The preliminary products produced by hot rolling these steels have a pearlitic structure with embedded cementite due to the high carbon contents, the pearlite being lamellar. Good cold formability, in particular good workability by punching, can only be achieved with these steels if the strength is significantly reduced. This is done by soft annealing for several hours in the temperature range between 650 and 730 ° C. The lamellar pearlitic cementite is converted into a spherical shape. The larger the particles of the molded cementite become, the lower the strength. A tempering treatment is then carried out on the finished parts produced from these steels to set the required strength and hardness values. After hardening and tempering, these steels have a high wear resistance due to their high proportion of spherical cementite.

From the specialist book by L. E. Samuels "Optical Microscopy of Carbon Steels", American Society of Metals, 1980, pages 225 to 246, in particular pages 231 and 245, it is known that graphite can be formed in hypereutectoid steels. This usually happens with soft annealing. The graphite formation occurs most strongly in the temperature range from 620 to 680 ° C. In this temperature range, as can be seen from the aforementioned specialist book, the speed for graphite formation is maximum.

The graphite can also occasionally be used to cool the primary material, e.g. B. slabs, blocks or hot strip and wire rod arise. A direct consequence of graphite formation is a reduction in strength and an improvement in cold formability, which can be used advantageously for many purposes (DE-PS 3934073).

However, graphite formation in hypereutectoid steels has a disadvantage. Elevated austenitizing temperatures of 850 ° C and higher and times of at least 10 minutes are required to dissolve it before hardening. This means increased energy consumption and a reduction in the operating performance of the hardening furnaces. Furthermore, due to the higher austenitizing temperature, a stronger scaling of the preliminary products produced can be expected. At lower austenitization temperatures, if the graphite dissolves incompletely, the hardness values are reduced after hardening. The incompletely dissolved soft graphite can also result in reduced wear resistance on finished parts such as knives and razor blades, which can lead to major material failures. To reduce the susceptibility of steels to graphite formation, it is known from "Journal of the Iron and Steel Institute", 203 (1965), pages 146 to 151 that for this purpose both the aluminum and the silicon content have to be kept very low. Aluminum plays a major role in the nucleation of graphite. Both Al₂O₃ and AlN serve as potential nuclei for graphite formation. The aluminum oxides are already forming during the solidification of the melt and remain largely unaffected by the hot forming process. Aluminum nitride separates before the graphite either during cooling from the rolling temperature or during annealing in the temperature range from 620 to 680 ° C, thereby favoring the cementite / graphite conversion. For this reason it is necessary to set the aluminum content below 0.010%. Aluminum levels below 0.005% are difficult to ensure with fully calmed steels.

In addition to aluminum, silicon has the greatest graphite-promoting effect. It causes an increase in the Ac1 temperature and a decrease in the stability of the cementite. The increase in the Ac1 temperature accelerates the carbon diffusion to the graphite nuclei, and the low cementite stability is reflected in a rapid course of the cementite / graphite conversion. For this reason, very low silicon contents must be aimed for. However, since silicon, like aluminum, is required to calm the steel, the silicon content cannot be reduced below 0.15%.

In contrast to aluminum and silicon, the manganese content counteracts graphite formation by stabilizing the cementite. It lowers the Ac1 temperature and at the same time the carbon activity by dissolving in the cementite. An increase in the manganese content stands in the way of its strong effect on the gamma-alpha conversion. As the manganese content increases, the conversion range is shifted to lower temperatures, which in the case of steels over 1% Mn leads to a tendency to form bainite when cooling after hot rolling. After soft annealing, the manganese-rich steels have a finer distribution of the molded-in cementite, which is associated with an undesirable increase in strength. It is believed that the annealed steels show a yield strength increase of 50 N / mm² per 1% Mn.

In addition to increasing the manganese content, the addition of chromium is probably the best known measure to increase the resistance to graphite formation. Chromium, as a strong carbide former, also accumulates in the cementite and stabilizes it. This reduces the driving force of graphitization. There are also limits to the alloying of chromium in carbon steels. The soft annealed steels show an increase in yield strength of around 200 N / mm² per 1% Cr, which results from an increased tendency of these steels to form bainite during cooling. In order to increase the resistance of a steel with graphitization above 0.9% C, a minimum content of 0.15% Cr must be ensured.

In "The Iron Age", August 1950, pages 64 to 68, it is mentioned that tellurium in cast iron is classified as a strong carbide stabilizer. However, the effectiveness of tellurium in steels has not been checked, since it was apparently assumed that it volatilizes at high temperatures.

Tellurium was first used as an alloying element in steel in the 1970s. DE-PS 2951812 describes a free-cutting steel containing tellurium with 0.002 to 0.1% tellurium. The task of the tellurium is to change the shape of the sulfidic inclusions and thus to increase the machinability of the steel. Free-cutting steel has carbon contents of up to 0.6% and sulfur contents of between 0.04 and 0.4% and chromium contents of 0.10 to 0.30%. The influence of tellurium on graphite formation was not discussed in this document. Another easily machinable steel with tellurium, which still contains a small amount of calcium, has been described in DE-PS 2824803.

A steel can be derived from DE-PS 3721641 (claims 7 and 8) with 0.32 to 0.9% C, 0.20 to 1.50% Mn, up to 2.0% Si, up to 0.15% Al , up to 3.5% Cr, up to 0.05% S, up to 0.05% P, up to 0.02% N, up to 3.5% Ni, up to 0.5% Mo, up to 0.005% Te. Table 1 also contains a steel DI with 0.44% C, 0.68% Mn, 0.25% Si, 0.016% P, 0.005% S, 0.0042% N, 0.012% Al, 1.05% Cr, 0.014% Ni, 0.003% Mo and 0.002% Te. In the case of these steels, which are not hypereutectoid, the tellurium addition is only intended to influence the sulfide shape, while the Cr, Ni and Mo additions are said to increase hardenability.

The invention is based on the object of proposing temperable hypereutectoid steels with carbon contents between 0.91 and 1.3%, in the manufacture and further processing of which no graphite formation occurs either before or after soft annealing.

This object is achieved according to the invention by a steel composition according to claim 1. In this new steel composition, the manganese content is reduced to a max. In order to avoid an undesirable increase in strength and the associated reduction in cold formability. 0.80% and the chrome content to max. 0.30% limited. Aluminum levels of up to 0.010% and silicon levels of up to 0.30% are permitted for complete reassurance in steel. Surprisingly, it was found that adding tellurium in the range of 0.0015 to 0.1% strongly inhibits the tendency to form graphite if the tellurium to sulfur ratio is greater than 0.20. However, it was found that the tellurium only works if the graphite-promoting elements silicon and aluminum are kept at the specified low level and the manganese and chromium contents the aforementioned maximum values of 0.80% and 0.30%, respectively. do not exceed.

Preferred analysis areas result from the subclaims. Claim 2 shows that with a restricted carbon content of 0.91 to 1.1% and an adapted sulfur value of max. 0.020% of the tellurium content is preferably 0.0020 to 0.0060%, while according to claim 3 with a restricted carbon content of 0.91 to 1.1% and an adjusted sulfur content of max. 0.010% of the tellurium content is preferably 0.0020 to 0.0040%, the ratio of tellurium in each case to sulfur is greater than 0.20.

According to claim 4, the steel can also be alloyed with vanadium, titanium, zirconium and boron, vanadium and boron serving to increase the hardenability and titanium and zirconium for nitrogen setting and sulfide shape influencing. The steel may also be treated with at least 0.001% calcium to form sulfide inclusions.

The invention is explained in more detail below on the basis of exemplary embodiments. Table 1 contains an overview of the steel compositions. The steels listed under A to J were melted using the oxygen inflation process and used for hot and cold rolled soft annealed products such as strip and wire. Steels B, D, G and I fall under the invention. Steels C, E, H and J are not covered by the invention since they have no tellurium. Steels A and F are not covered by the invention because they have a too low Te / S ratio.

Table 2 shows an overview of the graphite surface area after soft annealing with a residence time of 50 h in the temperature range between 620 and 680 ° C. It is clear from this that the steels covered by the invention have no graphite. The comparison in Table 2 shows that graphite formation can only be avoided with the steel composition according to the invention. Steel J, which was protected against graphitization by a high manganese and chromium content, remains graphite-free. However, the steel J has a high strength because of a cementite distribution that is too fine and is therefore not suitable for further processing by cold forming, in particular by stamping.

Claims (5)

Graphite-free hypereutectoid hardened and tempered steel with (in mass%) 0.91 to 1.30% C 0.20 to 0.80% Mn
up to 0.30% Si
up to 0.01% Al 0.10 to 0.30% Cr
up to 0.040% S
up to 0.040% P
up to 0.020% N
at most 0.1% Ni
at most 0.1% Mo 0.0015 to 0.1% Te Balance iron and unavoidable impurities, the ratio of tellurium to sulfur is greater than 0.20. Steel according to claim 1, with (in mass%) 0.91 to 1.10% C 0.20 to 0.80% Mn
up to 0.30% Si
up to 0.010% Al 0.10 to 0.30% Cr
up to 0.020% S
up to 0.040% P
up to 0.014% N
at most 0.05% Ni
at most 0.03% Mo 0.002 to 0.006% Te Balance iron and unavoidable impurities. Steel according to claim 1 with (in mass%) 0.91 to 1.10% C 0.20 to 0.80% Mn
up to 0.30% Si
up to 0.010% Al 0.10 to 0.30% Cr
up to 0.010% S
up to 0.040% P
up to 0.014% N
at most 0.05% Ni
at most 0.03% Mo 0.002 to 0.004% Te Balance iron and unavoidable impurities. Steel according to one of claims 1 to 3, which additionally contains the following elements (in mass%): up to 0.10% V up to 0.04% Ti up to 0.15% Zr up to 0.01% B. Steel according to one of claims 1 to 4, which additionally contains 0.001-0.05% Ca to influence the sulfide shape.