NO811738L - PROCEDURE FOR AA TREATING SILICONE STEEL, CALCULATED FOR HOT DELAY - Google Patents

Description

The invention relates to a method for treating silicon-containing steel products in connection with hot rolling so that the poorer result of subsequent hot-dip galvanizing due to the silicon content in the steel is avoided.

Normal unalloyed or low-alloy steels, where silicon only occurs as an acceptable impurity, e.g. unsealed steel, is well suited for hot dip galvanizing. When such steel is dipped into a hot zinc bath, the steel surface is coated with a dense and solid layer of iron-zinc phases. The growth of this layer occurs by the iron diffusing across the layer to the now molten zinc bath, where it combines with the zinc to form further iron-zinc phases. Then. the determining factor is the supply of iron to the outer side of the existing layer, and this only happens by diffusion^ the layer thickness increases according to known laws only proportionally to the square root of time, implying a relatively rapid gradual increase in thickness. For the layer to grow to twice the thickness, • a quadrupling of the time is thus required. The advantages of this are tangible as the management of the heat delay process is significantly facilitated. When steel objects are taken up from the zinc bath, a thin film of pure zinc is left on the outermost surface, which thereby becomes dense and shiny. Very small amounts of iron pass out into the zinc bath itself, which is why the loss due to hard zinc formation is small. Hard zinc consists of a compound of iron and zinc that is deposited in the zinc bath.

In steel containing silicon, hot-dip galvanizing does not produce the fine surface mentioned earlier. Instead, the surface often becomes dull gray and uneven due to poor adhesion to the underlying layer of iron-zinc phase.

• This is probably due to the iron-zinc phase formed being porous and uneven. The growth of the coating here occurs proportionally to the immersion time, which makes the process difficult to control, and more sensitive to ••other conditions in the bath, above all temperature variations within the bath and in the submerged steel condition. Significant amounts of zinc also go

lost as hard zinc. Already below 0.05 wt% of silicon in the steel, these disadvantageous effects appear. However, the most powerful effects have been found within the range of 0.05 - 0.10 wt% silicon.

Various attempts have been made to avoid these disadvantages, partly by changing the conditions in the zinc bath, for example by varying the temperature and composition, partly by pre-treating the steel surface in different ways. The first type of precautionary measures has not led to practical solutions, and the second type of precaution has been far too costly to constitute practical solutions.

It has now surprisingly been shown that very good hot-dip galvanizing properties can be obtained in unalloyed and low-alloy steels containing silicon content critical for hot-dip galvanizing by delaying the cooling directly after the final hot-rolling within the temperature range that does not fall below 650°C. The condition for a good result is also that the glow shell formed during the final rolling is allowed to sit back, and that air access to the rolling surfaces during the delayed cooling is made impossible.

The invention will be explained in more detail with the help of some examples.

Example 1

The first■example is taken from an investigation on a laboratory scale.

Coupons, 95 x 25 mm, are cut from a wide strip, which is normally wound up at 600°C directly after hot rolling. The steel had the following analy seve kt% : 0.05% C, 0.05$ Si, 0.515? Mn, 0.0105? P, 0.017$ S, 0.006$ N, 0.05$ Al, as well as the rest Fe and normal impurities.

The coupons were taken from the inside of the tape roll, and the thickness of the oxide layer was measured to be 8-10 µm. A number of such coupons are struck into thin plate holes which are closed by folding, and this package is heated to different temperatures under varying pressures, whereupon they are taken out, stained, dipped in flux, and suspended in a bath of molten zinc at 460 °C for 8 minutes. After removal and free cooling in air, a cross-section through the respective coupon was examined. It was then found that thick porous coating layers (330 - 4-4-0<y>m) with a gray matte surface were achieved on

i.;: not heat-treated coupons,

heat-treated coupons where the oxide layer has been removed before the heat treatment,

coupons which, with the bx layer retained, were held at 675°C for 60 minutes.

Thin, dense coating layers (100 µm, almost independent of the immersion time) with a glossy surface were, on the other hand, formed in coupons which, with a retained oxide layer, were obtained at 750°C for 15 - 120 minutes.

Example 2

This example is also taken from a survey

on a laboratory scale.

Coupons were cut from a hot-rolled wide strip which, after hot-rolling, was wound up at 750°C and then allowed to cool unimpeded in air, in the same way as in the previous example. The steel had the following analysis in weight%: 0.09% C, 0.05$ Si, 0.61$ Mn, 0.011$ P, 0.014$ S, 0.006$ N, 0.004. Al, as well as the rest Fe of normal impurities .

The vouchers were treated in largely the same way. However, the annealing temperature and annealing times varied in a different way. As in the previous example, two different types of coating appeared: 1. Thick, approximately 400 µm, porous coating with a gray matt surface on coupons that had not been annealed. 2. Thin, approximately 8 µm, compact coating with a glossy surface on material that had been annealed at 750°C for 15 minutes

725°C » 30 "

675°C " 4 hours

The coupons that were annealed at 700°C for two hours showed a transitional appearance with both of the above-mentioned coating

types represented.

Example 3

The test series in example 2 was repeated. However, one side surface is ground. of the coupons down 1 mm below the original surface before the enclosure in the zinc bath. On all sanded down surfaces, the same thick, porous coating is obtained as on untreated coupons according to point 1 in example 2.

Example 4

A number of strips with largely the same analysis as in example 2 were final rolled in a hot rolling mill at approx. 750°C and wound very tightly on a tape reel. The rolls of tape were then lowered into a heat-insulated airtight container, and held there for at least 15 minutes. The temperature therefore did not drop below 700°C in any part of the tape rolls. After loosening, they were hot dipped galvanized, which gave them a very dense and hard and completely acceptable surface.

Example 5

Instead of, as in example 4, where the tape was wound very tightly to avoid contact with the surrounding air, a number of tapes were wound very loosely. To lower the oxygen potential in the surrounding atmosphere, hydrogen gas is flushed through the container and during the delayed cooling time. The same good properties of the coating layer were achieved by subsequent var mf or zinc.

Example 6

To achieve special mechanical properties of

the finished strip, the final rolling temperature was lowered. The aim was that subsequent winding should take place at a temperature of the strip which should be as close to 700°C as possible. A portion of the tightly wound bands was placed in a container for controlled cooling where the temperature did not drop below -650°C. The bands that are kept in the container for at least 2 hours,

subsequent hot-dip galvanizing gave a completely acceptable surface despite the fact that the silicon content in the starting material was 0.06$ Si.

Example 7

Parts of the material part according to example 6, partly stained in the usual way, partly steel brushed before the hot-dip galvanizing to remove the rest of the glow scale formed during the hot rolling. The result was that the hot-dip galvanizing gave an even better result than with the material that had not been surface-treated in this way.

Example 8

The prerequisite for a good hot-dip galvanizing result is that the siliceous material is kept at at least 675 C for a certain time after hot rolling and without air access from the surroundings. In the case of hot-rolled wide strip, it has been shown that if the strips are wound tightly, this shuts out the air so effectively that you can maintain excellent hot-sinking results. When it concerns material in simple bar form, on the other hand, you are referred to closed containers, where the material is stored after hot rolling for delayed cooling. Experiments have been carried out where the rod material was stored in oxygen-shielded shelves, in air-tight tunnels and pockets with good results. The shelves are made heat-insulating to reduce heat loss to acceptable values.

Claims (5)

1 Process for producing unalloyed and low-alloy steel material with a carbon content that does not exceed 0.30 wt$ and contains 0.05 - 0.20 wt$ silicon, intended for hot-dip galvanizing, characterized in that the material is kept in the temperature range 675 - 850 after the final hot rolling °C for at least 15 minutes during which time the material, in connection with the final rolling of air oxygen slightly oxidized roll surfaces is prevented from. further oxidation is caused by preventing air access to said surfaces. 2. Method according to claims 1 and 2, characterized in that the supply of air to the material surfaces is prevented by the ambient air being replaced by a gas other than oxygen. 3. Method according to one of claims 1 or 2, characterized in that the material consists of steel bands and that these are coiled up so tightly that air is prevented from accessing the plate surfaces. U- Method according to claims 1-3" characterized in that the material is stained before hot-dip galvanizing to remove any oxide. 5. Method according to claims 1-3" characterized in that any oxide on the material surface is removed by means of steel brushing before the hot-dip galvanizing.