DE2823366A1 - Soft free-cutting steel contg. large sulphide inclusions - where controlled deoxidation of molten steel in ladle ensures good surface finish during machining - Google Patents

Abstract

The inclusions have the shape of a lens, but the semi-finished prods. machined from the steel have a good surface finish obtd. by measuring the oxygen content of the molten steel in the ladle and then injecting a deoxidant to reduce the oxygen to teh deisred percentage. The oxygen content of the molten steel is pref. measured via electromotive force; and the dissolved oxygen in the melt is pref. reduced to 30-130 ppm, esp. 40-60 ppm, prior to casting. The pref. deoxidant is Al or its alloys; and deoxidn. is pref. controlled via the equation log(O) = -20600/T + A; where (O) is the dissolved O2 in ppm before casting: T is the steel temp. in degree K; and A is a function of the initial O2 content and the type and amt. of deoxidant used plus the injection time. A good surface finish can be obtd. despite the presence of numerous large sulphide inclusions in the steel.

Description

Process for the production of free cutting steel

The invention relates to a method for the production of soft car steel with coarse lenticular sulphide inclusions with good surface quality at the same time of the semi-finished product.

Draw soft free-cutting steels, e.g. grades 9SMn28 and 9SMn36 is known to be due to lower C-contents (# 0.10%) and high S-contents (0.3 - 0.4%). The Mn content is 1.0 - 1.5% in adaptation to the S content. Al or Si may only be detectable in traces.

A special feature of these types of steel are their excellent machining properties properties.

However, the high machinability of free-cutting steel is not the only thing given by compliance with the analysis specification. Rather, this becomes essentially determined by the formation of the manganese sulphides precipitated in the steel. Aimed for lenticular inclusions are as coarse as possible.

The formation of sulphides is a function of that contained in the steel block Total oxygen content, predominantly as the Mno-Feo phase in the investigated Sample is present.

If a certain total oxygen content of approx. 0.020, a good sulphide formation is achieved, but as a result of an increased inclination not necessarily a satisfactory surface quality for marginal blistering of the semi-finished product A high cleaning effort and rejects are the result. Will the melts but more strongly deoxidized in the steelworks, i.e. to values far lower than 0.020 % 0, insufficient sulphide formation is to be expected. A controlled one Setting of oxygen values, which a favorable sulfide formation with at the same time result in a good surface, was not previously known.

The invention is based on the object of a method for production from machine steel with coarse lenticular sulfide inclusions at the same time to develop a good semi-finished product surface due to good reproducibility The results of a review of the sulphide formation are omitted or on random samples can be restricted.

This object is achieved according to the invention in that the oxygen content the melt in the ladle is measured before the pouring (1 afterwards by rinsing in a deoxidizer is brought to the desired value.

It is advantageous if the determination of the oxygen switch by measurement (EMK measurement) takes place, which is known only to the dissolved oxygen content captured in the melt.

The dissolved oxygen content of the melt is 30 before casting -130 ppm, preferably 10-60 ppm The foregoing was already pointed out that so far as a measure of the degree of deoxidation after the total oxygen content in the block determined by the extraction method was used. A better one According to the invention, the measure of the degree of deoxidation of the melt is the measurement of the dissolved oxygen content by means of EMF measurement in the still liquid steel. Investigations showed a good connection between the EMF measurement and the Sulphide index determined on the semi-finished product (see Figure 1). This is one Internal key figure that takes into account the number, size and shape of sulphides. The higher the numerical value, the finer and more elongated the sulfides are. Above the melt release is determined by the key figure averaged from the edge and center of the block a foot test. Melts with a code above 140 must be devalued.

The blowroom output is a measure of the surface quality.

The quality of the surface and sulphide formation run in opposite directions. Both good sulfides can only be achieved if a certain oxygen range is maintained formation as well as surface properties can be achieved.

It was found that for the control of the solved Oxygen content not only the amount of deoxidizing agent but also the expected decrease in temperature during the flushing process must also be taken into account got to.

As the melt cools down in the ladle, the solubility of oxygen in the steel decreases according to lg E03 = -20600 + A, T where = = the dissolved oxygen content in the steel before casting T = the steel temperature in kelvin A = a function of the initial oxygen content in the steel before ladle deoxidation, the type and amount of deoxidizing agents added and the duration and intensity of the rinsing.

Figure 2 is for melts with the same addition of deoxidizer an average temperature dependence of the dissolved oxygen content during tapping (ppm) according to the function cited above.

At the same temperature, the rinse brings an increase in the dissolved Oxygen content.

During operation, when controlling the dissolved oxygen content unifying the temperature influence (see Fig. 3) but taken into account in the type that the oxygen depletion in the steel is linearly related to the corrective deoxidizer rate can be specified, with the expected decrease in temperature during the correction treatment is represented as an additional parameter according to = = X D + Y AT - Z Here in mean A O decrease in the gel. Oxygen content in ppm D amount of the added Correction deoxidizer in kg / t X Constant that determines the application of the deoxidizer takes into account #T Expected decrease in steel temperature during the correction in ° C Y constant that takes into account, among other things, the level of the measured output temperature Z Constant that takes into account the duration and intensity of the flushing (see Fig. 4) With With the help of one tailored to the initial oxygen content and the expected decrease in temperature The amount of subsequently flushed in deoxidizing agent can be used to determine the oxygen content Set sufficiently precisely in the free-cutting steel. When set to 40-60 ppm Oxygen levels are obtained both with regard to the sulfide formation on the vehicle excellent results as well as the surface properties (see Fig. 4).

For setting the required oxygen values in liquid steel it is necessary to add special deoxidizers to the pan. After more general The view is the use of the conventional deoxidizing agents silicon and Aluminum in large quantities in soft free cutting steel is dangerous because the formed and in Deoxidation products remaining in steel, SiO2 and Al2O3 are particularly hard and the tool wear during machining is disadvantageous Way increase. Nevertheless, according to the invention, aluminum should be used as the deoxidizing agent or aluminum alloys are flushed in.

It was found that with small additions of aluminum which are not sufficient to feed the steel to an analytically verifiable aluminum content alloy, the formed alumina (Al2O3) is fully deposited so that the there are no adverse consequences for the machinability. When setting solved Oxygen levels in auto-steels of more than 30 ppm are a verifiable risk Achieving aluminum gels in steel is not yet possible.

Blank page

Claims (5)

P a t e n t a n s p r ü c h e 1. Process for the manufacture of soft Authorized steel with coarse, lenticular sulphide inclusions and at the same time good The surface texture of the semi-zoug, which is not shown t that the oxygen content of the melt in the ladle is measured before the pouring then brought to the desired value by flushing in a deoxidizer will. 2. The method according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the dissolved sanitary substance content of the melt is correct with the help of the EMF measurement will. 3. The method according to claim 2, characterized in that the dissolved Oxygen content of the melt before casting to 30 - 130 ppm, preferably 40 - 60 ppm is set. Claim 4. The method according to claim 1, 2 or 3, characterized in that for control the oxygen content the influence of the temperature decrease occurring during the rinsing process is taken into account on the oxygen vulnerability in steel according to the following function: lg [° 3 = -20600 + A, T means [0] = the dissolved oxygen in the steel the casting in ppm T = the steel temperature in Kelvin A = a function of the initial oxygen content in the steel before ladle deoxidation, the type and amount of deoxidizing agents added as well as the rinsing duration and intensity or # [o [o] = X. D + Y. A T - Z mean = Decrease in the gel. Oxygen content in the steel in ppm D = amount of added Correction deoxidizer in kg / t X = constant that determines the output of the deoxidizer takes into account # T = expected decrease in steel temperature during the correction in % Y = constant that includes the level of the measured output temperature takes into account Z = constant; which takes the winding duration and intensity into account 5. Method according to one of the preceding claims, characterized in that as Deoxidizer aluminum or aluminum alloys are flushed in.