GB2135336A - Hot deoxidation of semifinished metal tubes - Google Patents

Abstract

The inside wall of axially pierced round blanks intended for subsequent hot working in a long-mandrel rolling mill is deoxidized by a process which comprises the successive steps of continuously introducing into the blank, for a period of time of between one and ten seconds, a flow of powdered deoxidant, preferably consisting of borax, and an inert gas, preferably nitrogen, as vehicle for the deoxidant, and then interrupting this flow and feeding into the same blank a washing flow preferably consisting of a stream of the same inert gas as was used in the first step.

Description

SPECIFICATION Process for hot deoxidation of semifinished metal tubes The present invention relates to a process forthe deoxidation of semifinished metal tubes at temperatures of from 1000 to 1300 C. More particularly, this invention concerns a process for the internal deoxidation of axially pierced round blanks intendedforsubsequent rolling on an inner mandrel forthe production of seamless tubes.

One ofthe major requirements in the hot manufacturing of seamless tubes is the need for the deoxidation, that is to say the removal of the metal oxides which are unavoidably formed on the surfaces ofthe substrate to be processed when in contact with air, whether a billet, an axially pierced round blank or a tube is concerned. These oxides give rise to disadvantages, the nature and importance ofwhich are well known to those skilled in the art.This requirement is particularly applicable to axially pierced round blanks which are to be rolled on long mandrels onwhich they are previously mounted.

When the deoxidation is to be effected on the inside wall ofthe pierced blank, the difficulties increase to such an extent that in almost all cases compromise solutionsare accepted.

At the presenttime the deoxidation ofthe inside wall of pierced blanks is effected with pressurized water (at pressures between 20 and 200 bars) sprayed by means of suitable nozzles inserted within the pierced blank. This process, however, is complex, shows dimensional limits (minimum diameter of bore and maximum length of the pierced blank), is timeconsuming and causes reductions ofthetemperature ofthe blankwhich are notacceptableforthe subsequent rolling operations.

The main object of the present invention is to provide a method of deoxidation of axially pierced blanks to be processed in a long-mandrel rolling mill for the production of seamless tubes, which is ableto achieve rapid, effective removal of any metal oxides present on the inside wall ofthe pierced blanks.

This and otherobjects,which will be seen more clearly from the description given below, are achieved with a process according to the invention, which comprises the following successive steps: a) continuously feeding into the blank, for a period of time of between 1 and 10 seconds, a flow of powdered deoxidant and an inert gas as a vehicle for the deoxidant, so as to fluidize the metal oxides present inside the blank; b) interrupting this flow and feeding into the blank a washing flow, preferably consisting of a stream of the same inert gas as Was used as vehicle for the powdered deoxidant, so as to remove the previously fluidized oxides while atthe same time preventingthe furtherformation of oxides.

The powdered deoxidant is advantageously composes of borax (penta- or decahydrated sodium tetraborate) with suitable additives for achieving the chemicophysical properties required forthe purpose.

The inert gas used asvehicle forthe deoxidant and for the subsequent washing is advantageously nitrogen.

The time of feeding the deoxidantand nitrogen flow is strictly linked to the amount of powdered deoxidant which has been calculated to be sufficient to solubilize the amount and consistency ofthe total oxides which are known or supposed to be in the blank which is to be deoxidized.

These amounts are obviously average statistical values calculated on the basis of a relatively large number of experimental values.

It is been found that amounts of powdered deoxidizer between 0.5 and 0.7 kg per 1000 keg of pierced blank give excellent results. Such an amount of deoxidant is advantageously applied within a period of between 1 and 10 seconds, preferably between 1 and 5 seconds.

The nitrogen vehicle is given the task of carrying the powdered deoxidant and ensuring optimum diffusion ofthe latter overthe entire insidewallofthe blank.

For this purpose it has been found that the best results areobtained by using from 600 to 800 liters of gaseous nitrogen per tonne of blanks to be treated, this nitrogen preferably being fed at a pressure of from 3to 3.5 bars.

The same nitrogen, underthe same pressure, is then usedforthe blank washing step, which can be completed within a period of 2 to 5seconds.

Apparatus and/or devices suitable for supplying metered amounts of powdered deoxidanttaken from one or more tanks are known and widely available in industry, while apparatus and devices for the metered feed of gaseous nitrogen from tanks of liquid nitrogen -are likewise known.

Devices are also widely available in industryforthe monitoring,safety,timing, control and automation of the steps ofthe process according to the present invention, and will therefore not be described since they do notform an integral partofthe invention According to anotherfeature of the invention, the flow of powdered deoxidant and nitrogen is supplied immediately atthe inlet ofthe pierced blank which is to be deoxidized.

To this end the amount of deoxidant previously metered is mixed with the gaseous nitrogen in a venturi-type ejector. The latter imparts to the resulting mixture a speedsufficientto enable it to be delivered to one or more outlet nozzles and to be blown into the pierced blank, without the need of inserting the nozzle into the bore of the blank itself.

Since an axially pierced blank must be subjected to the deoxidation treatment before itis mounted on a mandrel of a seamless tube rolling mill, all the equipment and devices provided for carrying outthe process of the present invention are said to be a "deoxidation station" positioned between a billet piercing machine and the apparatus for inserting the mandrel into the pierced blank in a seamless tube manufacturing plant.

EXAMPLE Other characteristics and advantages of the invention will be seen more clearly from the following example, which is given without being a limitation.

Chemical composition by weight ofthe deoxidant product: Decahydrated borax Na2B407.10H2O 56% Fatty acids (soaps) 12% Sulfates expressed as Na2SO4 32% - Characteristics ofthe pierced blank: Material Steel Temperature 1250"C Outside diameter 350 mm Inside diameter 290 mm Length 6500 mm Weight 1540kg - Amount of deoxidant used: 0.9kg per blank equivalentto 0.58kg pertonne -- Amount of gaseous nitrogen used forthe insufflation ofthe mixture: 1100 1 per blank equivalentto 7141 pertonne for washing: 16001 per blank equivalentto 10391 pertonne Total 27001 per blank equivalentto 17531 pertonne - Mixtureinsufflationtime: 1.spec.

- Washing time: 2 sec.

Claims (8)

1. A process forthe hot deoxidation of semifinished metal tubes, particularly the inside wall of axially pierced round blanks intended for subsequenthotworking in a long-mandrel rolling mill, which comprises the following successive steps: a) continuously feeding into an axially pierced blank, for a period of time of between 1 and 10 seconds, a flow of powdered deoxidant and an inert gas as a vehicleforthe deoxidant, so as to fluidize the metal oxides present inside the blank, and b) interrupting this flow and feeding into the blank a washing flow,wherebythe previouslyfluidized oxides are removed, while atthe same time preventing the furtherformation of oxides.

2. A process as claimed in claim 1, wherein said washing flow in step b) consists ofthe same inert gas as was used in step a) as vehicle forthe powdered deoxidant.

3. A process as claimed in claim 2, wherein the deoxidant is composed of borax (penta- or decahydrated sodium tetraborate) with suitable additives.

4. A process as claimed in claim 2, wherein the inert gas is nitrogen.

5. A process as claimed in claim 4, whereinthe continuous flow of powdered deoxidant and gaseous nitrogen is maintainedfora period oftime of between 1 and 10 seconds, preferably between 1 and 5 seconds.

6. A process as claimed in claim 4, wherein the mixture of powdered deoxidant and nitrogen is formed in a venturi-type ejector capable of imparting to the mixture a speed sufficientto deliver itto at least one outlet nozzle positioned immediatelyattheinlet ofthe axially pierced round blankwhich is to be deoxidized but not inserted into the blank.

7. A process as claimed in claim 6, wherein the mixture comprises powdered borax metered in amounts between 0.5 and 0.7 kg per kg of blank, and gaseous nitrogen fed in metered amounts between 600 and 800 liters per 1000 kg of blankto be subjected to the treatment, the nitrogen being in addition supplied art a pressure of 3-3.5 bars.

8. Aprocessasclaimed in claimBwhereinthe continuous flow of nitrogen forthe blank washing step is maintained for a period of time of between 2 and 5 seconds, the nitrogen being supplied at a pressure of 3-3.5 bars.