SE446882B - Method for production of non-alloy and low-alloy steel with carbon levels not exceeding .30 weight percent and containing 0.05 to 0.20 weight percent silicon - Google Patents

Abstract

Non-alloy and low-alloy steel endure a deterioration of hot galvanization properties at a silicone level as low as 0.05 weight percent. To counteract this deterioration, the oxide film on the steel's surface at the final warm rolling stages is allowed to remain and cooling in a temperature range from 675-800 degrees Celsius is delayed for a certain time depending on the temperature, at the same time as oxygen from the outer air is kept from contacting the surface of the steel.

Description

15 20 25 30 35 446 882 '2 especially temperature variations within the bath and in the immersed steel object. Significant amounts of zinc are also lost as hard zinc.

Concentrations already below 0.05% by weight of silicon in the steel produce these unfavorable effects. However, the strongest effects have been found in the range 0.05 - 1.0% by weight of silicon.

Attempts have been made in various ways to avoid these inconveniences, partly by changing the conditions in the zinc bath, for example by varying the temperature and composition, and partly by pre-treating the steel surface in different ways. The first type of measures have not led to practical solutions and the latter type of measures have been too costly to constitute practical solutions. SUMMARY OF THE INVENTION: It has now surprisingly been found that very good hot-dip galvanizing properties can be obtained in unalloyed and low-alloy steels containing critical silicon contents for hot-dip galvanizing by delaying the cooling within a temperature range of not less than 650 ° C immediately after the final hot rolling. The prerequisite for a good result is also that the scale formed during the final rolling must remain in place and that air access to the rolling surfaces during the delayed cooling is made impossible.

The invention will be described in more detail here with the aid of eight examples and in the appended claims.

EXAMPLE 1: The first example is taken from a laboratory-scale study.

Coupons, 95 x 25 mm, were cut from a broadband, which is usually picked up at 600 ° C immediately after hot rolling. The steel had the following analysis in weight percent: 0.05% C, 0.05% Si, 0.5l% Mn, 0.010% P, 0.017% S, 0.006% N, 0.05% Al and the rest Fe and normal impurities .

The coupons were taken from the inside of the strip roll and the thickness of the oxide layer was measured to 8 - 10 Fm. A number of such coupons were wrapped in thin plate casings which were added by folding and these packages were heated to different temperatures for varying times, after which they were taken out, pickled, dipped in flux and hung 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 - 440_pm) with a gray matte surface were obtained on 10 lS 20 25 30 35 446 882 - non-heat treated coupons - heat treated coupons where the oxide layer was removed before heat treatment - coupons which retained 6 layers of oxide C for 60 min Thin dense coating layers (approx. 100 μm, almost independent of dipping time) with a glossy surface were obtained, on the other hand, in coupons which with the retained oxide layer nauit at 7 50 ° C under ice - 120 min.

EXAMPLE 2: This example is also taken from a laboratory-scale study.

From a hot-rolled broadband which, after hot-rolling, was wound up at 70 DEG C., it was then completely removed in the same way as in the previous example. The standing had the following analysis in weight percent: 0.09% C, 0.05% Si, 0.6l% Mn, 0.0ll% P, 0.014% S, 0.006% N, 0.004% Al and the rest Fe and normal impurities. the coupons were treated in much the same way. However, the annealing temperatures and annealing times varied in a different way. As in the previous example, two different types of coating were obtained: l. Thick, approximately 400 μm, porous coating with a gray-matte surface on coupons that had not been annealed. 2. Thin, approximately 80 μm, compact coating with glossy surface on material annealed at 1so ° c under ice nin 1zs ° c "so" s7s ° c "4: in The coupons annealed at 700 ° C for 2 hours showed transition appearance with both types of coating described above represented.

EXAMPLE 3: The test series of Example 2 was repeated. However, one side surface of the coupons was ground down 1 mn below the original surface before being suspended in the zinc bath. On all sanded surfaces, the same thick, porous coating was obtained as on untreated coupons according to point 1 in Example 2.

EXAMPLE 4: A number of belts with substantially the same analysis as in Example 2 were finally rolled in a hot rolling mill at about 750 ° C and rolled very tightly on a belt reel. The tape rolls were then lowered into a heat-insulated airtight container and kept there for at least 15 minutes. The temperature then did not fall in any part of the belt rollers below 700 ° C. After pickling, they were hot-dip galvanized to obtain a very dense and hard and perfectly acceptable surface.

EXAMPLE 5: Instead of as in Example 4, where the tapes were wound very tightly to avoid contact with ambient air, a number of tapes were unwound very loosely. To lower the oxygen potential in the surrounding atmosphere, nitrogen gas was purged through the containers during the delayed cooling time. The same good properties on the coating layer were obtained on subsequent hot-dip galvanizing.

EXAMPLE 6: - In order to obtain special mechanical properties of the finished strip, the final rolling temperature was lowered. The intention was that subsequent winding would take place at a temperature of the belt, which would be as close to 700 ° C as possible. Some of the tightly coiled tapes were placed in a container for controlled cooling where the temperature was not intended to drop below 650 ° C. The strips that were kept in the container for at least 2 hours had a completely acceptable surface during subsequent hot-dip galvanization, despite the fact that the silicon content in the original material was 0.06% Si.

EXAMPLE 7: Parts of the material portion according to Example 6 were partly grazed in a conventional manner and partly steel-brushed before the hot-dip galvanizing to remove the rest of the scale formed during the hot rolling. The result was that the hot-dip galvanizing gave an even better result than with the material that was not surface-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 the hot rolling and without air supply from the surroundings. In the case of hot-rolled broadband, it has been found that if the belts have a hot-air location tightly, this excludes the air so efficiently that good hot-dip galvanizing results can be obtained. In the case of materials in the form of rods, for example, reference is made to closed containers where the material is stored after hot rolling for delayed cooling. Experiments have been made whereby rod material has been stored in oxygen-shielding enclosures, in airtight tunnels and pockets with good results. The covers have been made heat-insulating to reduce the heat losses to acceptable values.

Claims (5)

446_ 882 PATENTKRKV Methods for producing unalloyed and low-alloy steel materials with a carbon content not exceeding 0,30% by weight and containing 0,05 to 0,20% by weight of silicon intended for hot-dip galvanizing, characterized in that the material is kept within the temperature range 700 - 800 ° C after final hot rolling for at least 15 minutes during which time the material in connection with the final rolling of air oxygen slightly oxidized rolling surfaces is prevented from being further oxidized by preventing air access to said surfaces. 2. A method according to claim 1, 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. A method according to any one of claims 1 or 2, characterized in that the material consists of steel strips and that these are wound up so tightly that air is prevented from entering the sheet metal surfaces. 4. A method according to claims 1-3, characterized in that the material is pickled before hot-dip galvanizing to remove any oxide. 5. A method according to claims 1 - 3, characterized in that any oxide on the material surfaces is removed by means of steel brushing before hot-dip galvanizing.