FI72278C - Process for the production of non-alloyed and low-alloyed steel material a for hot galvanizing. - Google Patents

Description

1 72278

Method for the production of unalloyed and slightly alloyed steel material for hot-dip galvanizing

The present invention comprises a method for treating silicon-containing steel products in connection with hot rolling so that the degraded result caused by the silicon content in the steel is reversed in the subsequent hot-dip galvanizing.

Normal unalloyed or slightly alloyed steels in which silicon is present as an acceptable impurity, e.g. brittle steels, are well suited for hot dip galvanizing. When said steels are immersed in a hot zinc bath, the surface of the steel is covered with a dense and solid layer consisting of iron-zinc phases. The growth of this layer takes place so that the iron is dispersed throughout the layer until it reaches the molten zinc bath, where it coalesces with the zinc, forming new iron-zinc phases. Since the determining factor is the migration of iron onto the outer surface of the previously developed layer, and this occurs only through diffusion, according to known laws, the thickness of the layer increases only with respect to the square root of time, thus meaning a relatively rapid thickness increase. Thus, in order for the thickness of the deposit to double, the time must be quadrupled. The advantages of this are considerable, since the control of the hot-dip galvanizing process is substantially facilitated. When lifting steel objects up from the zinc bath, a thin film of pure zinc remains on the outer surface of the galvanizing, which makes the surface tight and shiny. Very small amounts of iron are transferred to the actual zinc bath, so the loss caused by hard formation is negligible. Hard zinc is formed from the central reaction of iron and zinc and settles in a zinc bath.

Steel containing silicon is not hot-dip galvanized to form the fine surface described above. Instead, the surface often becomes dull gray and uneven due to poor adhesion to the underlying iron-zinc phase. This is probably due to the fact that the iron-zinc Paasi in question is porous and uneven. When the growth of the coating in the process occurs in relation to the immersion time, this makes the process difficult to control and more sensitive to other conditions in the bath, especially to the temperature variation in the bath itself as well as in the immersed steel article. Significant amounts of zinc are also lost as hard zinc. Silicon contents in the steel as low as 0.05% by weight cause these adverse effects. However, the strongest effects have been observed in the range containing 0.05-0.10% by weight of silicon.

Various efforts 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 pretreating the steel surface in different ways. The former measures have not in practice led to the desired result and for the latter the costs have been far too high to be suitable for practical solutions.

SE 195734 relates to the heat treatment of steel for electrical purposes, in particular to a heat treatment which reduces the carbon content of the steel, anneals the steel, removes tension from the steel and coats the steel with a surface film having electrical resistance properties. In this case, the starting material is an oxide-free (glow-free) surface, which is then oxidized in the first treatment step by a special annealing treatment in which an oxidizing gas is added. During a separate annealing step, heat is added. A SiO coating 2 is formed in the final product, which would be a barrier to zinc attachment.

It has now surprisingly been shown that very good hot-dip galvanizing properties can be obtained in an unalloyed and slightly alloyed steel material with a carbon content not exceeding 0.30% by weight and containing 0.05-0.20% by weight of silicon by keeping the material in the final hot rolling mill. thereafter for at least 15 minutes in the temperature range of 675-850 ° C, during which time the weakly oxidized rolling surfaces during the final rolling of the material are prevented from further oxidizing by preventing air from entering said surfaces. A prerequisite for a good result is also that the annealing scale created in the final rolling must remain in place.

The invention is explained in more detail by means of eight examples and in the appended claims.

Example 1

The first example is taken from a study performed on a laboratory scale.

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In the usual manner, immediately after hot rolling at 600 ° C, 95 x 25 mm labels were cut from the wound plate strip. The analysis of the steel was as follows in weight percent: 0.05% C, 0.05% Si, 0.51% Mn, 0.010% P, 0.017% S, 0.006% N, 0.05% Al, and the remainder Fe and normal impurities.

The labels were taken from inside the tape roll and the thickness of the oxide layer was measured to be 8-10 μm. Several such labels were enclosed in thin sheet metal shells which were closed by bending and these packages were heated to different heat readings at different lengths of time, after which they were taken out, pickled, ..... . . . o Dipped in melt tea and hung in molten 460 c zinc bath for 8 minutes. After removal and free cooling in air, a cross-section was inspected through the corresponding label. It was then found that thick, porous, gray-matte coating layers (330-400 μm) had formed - not on heat-treated labels - on heat-treated labels from which the oxide layer had been removed before heat treatment 4 72278 - on labels with oxide layers 60 min.

Thin, dense, glossy coating layers (about 100 μm, almost independent of the immersion time) were instead obtained for labels with their oxide layers kept at 750 ° C for 15-20 minutes.

Example 2 This example is also taken from a laboratory scale study.

Of hot - rolled strip, which, after hot - rolling,. o was wound at 750 ° C and then allowed to cool unimpeded in air in the same manner as in the previous example. The analysis of the steel was as follows in weight percent: 0.09% C, 0.05% Si, 0.61% Mn, 0.011% P, 0.014% S, 0.006% N, 0.004% Al and residual Fe and common impurities.

The labels were treated in roughly the same way. However, annealing temperatures and annealing times were varied in a different way. As in the previous example, two different types of coatings were obtained: 1. A porous, gray-matt surface about 400 μm thick for labels that had not been annealed.

2. A thin, about 80 μm, dense, glossy coating for materials annealed at 750 ° C for 15 minutes at 725 ° C to 30 ° C for 30 hours.

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Labels that had been annealed at 700 ° C for 2 hours appeared to have transition surfaces in which each of the coating types described above was represented.

Example 3

The experimental series of Example 2 was repeated. However, the second side surface of the labels was ground 1 nun below the original surface before hanging in a zinc bath. The same thick, porous coating was obtained for all ground pimples as for the untreated labels according to Example 2 (1).

Example 4

A number of strips, the analysis of which was approximately the same as in Example 2, were final rolled in a hot rolling mill at about 750 ° C and wound very tightly on a strip spool. The Nau spars were then placed in a heat-insulated, airtight container and held there for at least 15 minutes.

The temperature of any part of these rolls of tape did not fall below 700 ° C. After pickling, they were hot-dip galvanized to obtain a very tight and hard, perfectly acceptable surface.

Example 5 In contrast to Example 4, where the strips were wound very tightly, several strips were wound very loosely to avoid contact with the ambient air. To reduce the oxygen potential of the ambient air, nitrogen gas was pumped through the tanks during slow cooling. Subsequent hot-dip galvanizing gave the coating layer the same good properties.

Example 6

In order to provide special mechanical properties to the finished strip, the final rolling temperature was lowered. It was intended that the next rewinding would take place on tape 6 72278

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at a temperature as close as possible to 700 C. Some of the tightly wound strips were placed in a tank for controlled cooling where the temperature was not intended to fall below o 650 C. The strips kept in the tank for at least 2 hours had a fully acceptable surface in the next hot dip galvanizing. 06% Si.

Example 7

Parts of the batch of material were pickled according to Example 6 in a partially conventional manner, in part the steel was brushed before hot-dip galvanizing to remove the residue from the annealing scale formed by hot rolling. As a result, hot-dip galvanizing gave an even better result than the material whose surface had not been treated in these ways.

Example 8

The prerequisites for a good hot-dip galvanizing result are that the silicon-containing material is kept at at least 675 ° C for a certain time after hot rolling and its contact with the surrounding air is prevented. With regard to hot-rolled sheet strips, it has been found that if the strips are wound tight, air is excluded so effectively that perfectly acceptable hot-dip galvanizing results can be achieved. In the case of, for example, rod-shaped material, recourse must be had to closed containers in which the material is stored after hot-rolling for slow cooling. Experiments have been performed in which the fabric material has been stored in oxygen-insulating coatings, airtight tunnels and protective pipes with good results. The covers are made of heat insulators to reduce heat losses to acceptable values.

The examples show that a thick, porous coating layer, normally formed in hot-dip galvanizing of steel containing 0.05-0.20% by weight of silicon, can be avoided by holding this material immediately after hot rolling for two hours or longer at 675-850 ° C: in. If the temperature was maintained at 700-800 ° C, the residence time could be significantly reduced. During this period, the surfaces must be prevented from further oxidizing by preventing air from entering said surfaces. This can be done either by winding the strip material so tightly that air ingress is prevented or by removing the ambient air with a non-oxidizing gas. A better result could be obtained in the subsequent hot-dip galvanizing if the material was first pickled or brushed with a wire brush to remove the oxide residue on the strip.

Claims (3)

A process for the production of non-alloyed and lightly alloyed steel material for hot-dip galvanizing with a carbon content not exceeding 0.30% by weight and containing 0.05 to 0.20% by weight of silicon, characterized in that the material is kept low after final hot rolling. o for up to 15 minutes in the temperature range of 675-850 ° C, during which time the weakly oxidized rolling surfaces during the final rolling of the material are prevented from oxidizing for 1 i by preventing air from entering said surfaces. Method according to Claim 1, characterized in that the entry of air into the material surfaces is prevented by replacing the ambient air with a gas other than oxygen. A method according to claim 1 or 2, characterized in that the material is formed by steel strips and that these are wound so tight that air does not enter the plate surfaces.