DD156988B1 - PREPARATION OF A TI-MICRO-ALLOY PAYMENT STEEL - Google Patents

Description

Field of application of the invention

The invention relates to the production of a Τΐ-micro-hardened tempering beam for thick-walled cast steel products.

Characteristic of the known technical solutions

In the production of steel melts for tempered steel casting in an electric arc furnace, aluminum or Al-containing alloys are used for the deoxidation. Due to different deoxidation conditions, the yield varies significantly in the aluminum, so that the Al residual levels control very strong. The deoxidizing agent aluminum has the property of being dissolved in the steel elements, .Z. As nitrogen, to react, whereby submicroscopic precipitates in the structure and on the grain boundaries can arise. Heat-treat steels also contain higher levels of carbide-forming elements, such as: S. Manganese, chromium and molybdenum. Reaction of the carbon with these elements can also lead to submicroscopic precipitates at grain boundaries upon cooling. The reaction of the residual aluminum with the nitrogen or the carbon with the carbide-forming elements is the more intense the slower the cooling rate or the greater the cooling modulus of the casting. The submicroscopic precipitates represent sharp notches, which under tension or stress can lead to the formation of high concentrations of stress or to breakage in this area. In practice, to avoid submicroscopic aluminum nitride precipitations, high quenching rates, defined Al contents or Ti contents of 0.05% are proposed. (44th International Foundry Congress Florence 1977 v. KEHöner and S. Baliktey, p. 138) In practice, such precipitates can not or only insufficiently avoid, so that when cooling due to the stresses occurring preferential cracks in thick-walled castings arise, the heavy Damage to these castings may result. It is known from the German periodical magazine "Foundry", 1979, pages 35 to 39, that Ti contents above 0.04% are as embrittling as aluminum nitride precipitations The use of a weldable, higher strength and vibration stressed steel filament for steel castings is known from the FRG patent The steel alloy has the following composition: 0.02 to 0.15% C, 0.10 to 1.0% Si, 0.10 to 2.5% Mn, 0.01 to 0.2% Al, 20 -200 ppm nitrogen, 0.01 to 0.1% niobium and tantalum, 0.01 to O, 2% V, 0.01 to 0.2% Ti, 0.1 to O, 2% Zr, remainder iron with conventional impurities This alloy is intended to be used to produce cast steel products which, after solution treatment, have a yield strength of more than 40kp / mrn ² , a notched impact strength greater than 6kpm / mm ² and a fatigue endurance ratio (at unnotched edge specimens) Oo / Oe greater than 0.45 Laid-open specification 2517164 (addition to the patent 22 39 092), this alloy is also for dickwandi used in cast steel products. Thereafter, in the interior of the thick-walled precipitation-hardened steel castings with air cooling of solution temperatures with minimum yield strengths of 30kp / mm ^2, a significant increase in strength over a non-microalloyed comparative steel should be achieved. In the DE-OS 2239092, page 2, last paragraph, it is noted that the embrittlement phenomena in castings, in particular by Al-N precipitates, ultimately can only be avoided by the application of solution annealing at temperatures above 1200 ⁰ C. Nitrides and carbonitrides of the micro-alloying elements are considered to result in similar grain boundary damage as Al-N precipitates. This statement is confirmed in the reference "foundry" 1979, pp 35-39, according to which Ti contents of 0.04 and 0.05% for a tempering steel ,. which is similar in its composition of the claimed alloy, also acts as besprödend such as Al-N precipitates. These precipitates, however, can not have a damaging effect when the grain boundary substances destroyed by a hot forming or be arranged in rows or as set forth in the Patent DE-OS 2239092, a solution heat treatment in the range from 900 ⁰ C to 1150 ° C and then isothermal annealing is performed.

DE-OS 2408721 describes a low-alloyed nitrogen steel having N contents of between 0.01 and 0.02% for shaped castings which are subjected to a heat treatment. When using the alloy steel in the composition of 0.16-0.22% C, 0.2-0.6% Si, 0.8-1.6% Mn, 0-1.6% Ni, 0-0, 4% Mo, 0.01-0.02% N, 0.1-0.2% V and / or 0.02-0.1% Ti, remainder Fe and common impurities, the mechanical properties only after a wall thickness independent Normalization annealing achieved. All known solutions of the prior art achieve the reduction of the risk of cracking only together with other mechanical properties after an additional heat treatment.

Object of the invention

The aim of the invention is to limit the risk of cracking, which is particularly in the production of thick-walled cast steel products made of tempered steel, and to increase the service life of the castings.

Explanation of the essence of the invention

The object of the present invention is to reduce the precipitation of ultrafine submicroscopic particles with slowly cooling thick-walled castings by means of metallurgical measures.

According to the invention the object is achieved in that titanium or Ti-containing alloys are added to the melt in the oven, in the pan, in the pouring stream or in the still liquid casting in such a height that reduces an excretion of submicroscopic particles so that no structural damage occurs. Setting an optimum Ti content limits the amount of submicroscopic precipitates of aluminum nitrogen compounds as well as manganese, chromium and molybdenum carbides, and precipitates microscopically visible coarse Ti carbonitrides having favorable metallurgical properties. The composition of the tempered steel for wall thicknesses> 80mm

0.20-0.50% C 0.30-1.60% Ni

0.40-1.50% Mn 0.20-0.50% Mo

0.30-1.60% Cr

The balance iron and common impurities that in the final analysis, a Ti-content of 0.01 to 0.04%.

embodiment

The effect of the invention is described for example on a tempering steel of the following composition: 0.30% C, 0.30% Si, 0.70% Mn, 1.50% Cr, 1.50% Ni, Q, 2S% Mo, balance Iron and usual impurities.

During the tapping of the aluminum deoxidized molten steel, Ti or Ti-containing alloys are added in such an amount that final Ti contents are in the range of 0.01 to 0.04% in the final analysis. The addition of titanium ensures that mainly nitrogen but also carbon are bound as coarse titanium carbonitride and thus the extent of submicroscopic aluminum nitride and special carbide precipitations are reduced. The reaction of the titanium takes place in the melt and during cooling in the solidified casting up to a temperature range of about 500 ⁰ C. The experiments showed that when setting an optimum Ti content, the toughness properties of the microalloy heat-treated steel casting are not deteriorated.

Claims (1)

Invention claim: Production of a Ti-microalloyed cast steel alloy for thick-walled castings> 80 mm wall thickness with alloy components C, Mn, Cr, Ni, Mo, which are added in the furnace, in the ladle, in the pouring stream or in the still liquid casting, characterized in that Ti resp Ti alloys of molten steel in the composition 0.20-0.50% C 0.40-1.50% Mn 0.30-3.0% Cr 0.30-2.0% Ni 0.20-1.50% Mo be added so that in the final analysis, a Ti content of 0.01-0.04% is present.