DE19543804B4 - Process for producing hot-dip galvanized steel strip and hot-dip galvanized sheet or strip made of steel made therewith - Google Patents

Abstract

Process for the production of hot-dip galvanized steel strip with the following process steps:
- Production of a steel strip with good cold formability by hot rolling, reeling and subsequent cold rolling and recrystallizing annealing in the bell annealer
- Heating the steel strip thus produced, cooled to about room temperature, in the uncoiled state to a temperature between 420 ° C and 500 ° C with a heating rate of 150 ° C to 300 ° C / s
- Immediately thereafter, introducing the heated steel strip essentially without cooling into a zinc melt bath having approximately the same temperature
- Subsequent cooling with a cooling unit and dressing the hot-dip coated tape.

Description

The invention relates to a method for the production of hot-dip galvanized steel strip. The invention relates also a hot-dip galvanized, cold-rolled and age-resistant sheet or steel band.

To DE-OS 1 521 376 it is known to heat steel strips to 450 to 500 ° C before introduction into the zinc bath after the strip has previously been annealed in a reducing atmosphere at a temperature of 720 to 950 ° C.

According to the same document, it is also known to use soft annealed strips in which no annealing treatment takes place. Instead, the strips are heated up to 230 ° C and pulled at this surface temperature through the zinc bath heated to 450 ° C. This method has the disadvantage that the zinc bath has to be heated from the outside by an uneconomical induction heating system and difficulties arise. It has therefore been proposed to continue annealing the strips softly, to clean them and to heat them in a reducing atmosphere to a temperature between 500 and 720 ° C. before the strip is introduced into the zinc bath at approximately 480 ° C. This should achieve a yield strength of only 20 kg / mm ² .

Annealing treatment at temperatures of more than 500 ° C before the introduction of the tape in the zinc bath is also in the more well-known state of the Technology as a basic measure intended.

For example, in DE-OS 1 621 340 the strip is annealed at temperatures between 650 and 750 ° C.

To DE-OS 23 40 413 the strip is heated to 750 to 800 ° C under non-oxidizing conditions before it is introduced into the zinc bath.

To DE 27 11 041 the strip is heated to 650 to 950 ° C before it is immersed in the zinc bath after cooling.

To DE 40 38 186 C1 can only be produced in a continuous process using the hot-dip process. After an annealing process, the usual aging treatment, in which the strip must be kept below 450 ° C for at least 60 seconds, must be carried out by cooling in three cooling phases, each with a defined cooling rate. The aim of this is to reduce the yield strength of the steels treated according to the invention and to increase the elongation at break. An improvement in the aging resistance is said to have been achieved on samples that were tested after a storage period of 4 weeks in order to take into account the usual storage times between production and processing. This beneficial effect is caused by a reduction in the harmful, dissolved carbon content achieved with the intended cooling. In this case, too, the recommended cooling with defined cooling rates is preceded by an annealing treatment of 750 to 800 ° C. It also shows that it is not an aging-resistant sheet with a high yield strength.

In contrast, aging-resistant sheets can be used by recrystallizing annealing in the bell annealer.

Heintz's article “Process engineering New developments in hot dip galvanizing "in BLECH 1967, no. 1, pages 5 to 12 clarifies that hot-dip galvanizing of steel sheets in regular contact with a continuous annealing has been made. In doing so, the metallurgical properties changed by cold-rolled and recrystallizing annealed sheets. In particular these sheets are not resistant to aging, which is why an aging treatment for such Sheet metal is known. A modified Cook-Norteman method is included in the article described in the pre-annealed and processed bundles are processed. Because that wound up in the bund Steel strip has already undergone recrystallizing annealing, the tape is processed in a preheater using the modified process warmed to zinc bath temperature, in a Sendzimir plant using an annealing furnace, which is only heated to the zinc bath temperature. The Material properties of the strip to be galvanized did not change, with what a difference to the recrystallizing one that completely changes the material annealing of the Sendzimir process. By warming the Steel strip, however, is created again in the process described Signs of aging that are not caused by the usual aging treatment can be eliminated because of the recrystallizing glow in the bundle caused and desired material properties would be significantly affected.

The present invention lies hence the task of producing a hot-dip galvanized steel strip to allow that its material properties produced by the recrystallizing annealing, especially its aging resistance, maintains even after the fire gating.

• – Herstellung eines eine gute Kaltumformbarkeit aufweisenden Stahlbandes durch Warmwalzen, Haspeln und anschließendes Kaltwalzen und rekristallisierendes Glühen im Haubenglühofen
• – Erwärmung des so hergestellten, auf etwa Raumtemperatur abgekühlten Stahlbandes im abgehaspelten Zustand auf eine Temperatur zwischen 420 °C und 500 °C mit einer Aufheizgeschwindigkeit von 150 °C bis 300 °C/s
• – unmittelbar darauf Einleiten des erwärmten Stahlbandes im Wesentlichen ohne Abkühlung in ein etwa die gleiche Temperatur aufweisendes Zink-Schmelzbad
• – anschließende Abkühlung mit einer Kühleinheit und Dressieren des schmelztauchbeschichteten Bandes.

This object is achieved according to the invention with the following process steps:

• - Production of a steel strip with good cold formability by hot rolling, reeling and subsequent cold rolling and recrystallizing annealing in the bell annealer
• - Heating the steel strip thus produced, cooled to about room temperature, in the uncoiled state to a temperature between 420 ° C and 500 ° C with a heating rate of 150 ° C to 300 ° C / s
• - Immediately thereafter, introducing the heated steel strip essentially without cooling into a zinc melt bath having approximately the same temperature
• - Subsequent cooling with a cooling unit and dressing the hot-dip coated tape.

The manufactured in the usual way Cold strip is essentially pre-immersed in the weld pool only heated to the molten bath temperature. In particular the temperature of the belt surface increased to no more than 500 ° C during galvanizing. The tape dives immediately after this warming into the weld pool. The advantage here is that both the soft unalloyed grades as well as the higher strength ones microalloyed grades the coated sheet is independent of the coating type the same chemical composition and have the same technological characteristics. The technological Properties of the coated steel strip are not only of the chemical composition of the steel depends, but also on the Heat treatment that has passed through the coated steel sheet. The invention of the usual so far continuous annealing in connection with a different galvanizing process leads to the significant advantage that the processing of the coated sheet no longer has to be adapted to the type of coating. So are the same technological properties with the same chemical Analysis independent achievable by the type of coating.

In contrast, when using a continuous annealing the strips for hot-dip galvanizing high temperatures are heated. To do the same technological To generate properties such as with annealed material (e.g. St 14) in connection with an electrolytic coating had to be achieved for hot-dip galvanizing is of a higher quality Material (e.g. IF steel).

In the method according to the invention it is no longer necessary to apply the galvanizing treatment to a continuous annealing to vote as a continuous annealing is not used. The galvanizing is therefore no longer dependent on recrystallization treatment in continuous annealing. Therefore the belt speed can no longer be matched to the steel grade. The performance of the galvanizing system is therefore only determined by designing the galvanizing part.

The method according to the invention is particularly suitable for steel strips which have a quasi-isotropic deformation behavior (<Δr value) and a high yield strength. Such a cold strip has the following analysis, for example:
0.02 to 0.06% carbon, preferably 0.03 to 0.048% carbon,
0.01 to 0.40% silicon, preferably up to 0.06% silicon,
0.10 to 0.80% manganese,
0.005 to 0.08% phosphorus,
0.0005 to 0.02% sulfur,
0.002 to 0.009% nitrogen,
0.015 to 0.08% aluminum,
0.01 to 0.04% titanium,
Max. 0.15% of one or more of the elements copper, vanadium, nickel, balance iron and unavoidable impurities.

The cold-rolled produced according to the invention hot-dip galvanized steel sheet or strip is resistant to aging.

The invention is explained below of embodiments for the production of a cold strip with extremely low anisotropy (Δr <0.2) explained in more detail.

From the melts A-D and the comparative melts E-F (Table 1), slabs 210 mm thick are cast in the strand. After warming up in the pusher furnace to 1250 ° C the slab was rolled into 3 mm thick hot strip, coiled and cooled to room temperature. The roll end temperatures and reel temperatures are shown in Table 2. After pickling, tapes became by cold rolling in different stages from 10% up to 80 % reduced to thin sheet thickness and re-coiled. The covenant was in the bell annealer to 700 ° C heated annealed with a throughput of 1.1 t / h to 1.9 t / h and then in Oven to 120 ° C cooled.

Cold rolling depending on the titanium content takes place at:
0.01% titanium with 20-60%, preferably 30-50% cold rolling degree;
0.02% titanium with 10-15% or 40-85%, preferably 50-80% cold rolling degree;
0.03% titanium with 10-20% or 50-85%, preferably 60-80% cold rolling degree;
0.04% titanium with 15-25% or 55-80%, preferably 20% or 60-70% degree of cold rolling.

For Intermediate values of the titanium alloy element, for example 0.025 % Ti, cold rolling grades up to 15% or 20% and up are preferred 85% set. Values between 0.01% and 0.02% are preferred lower cold rolling degrees set. The preferred cold rolling grade was assigned a high level of tensile strength and yield strength become.

• a) Kaltwalzgrad 10 bis 15 % = Streckgrenzenniveau R_p 0,2 = 400 bis 350 N/mm² Zugfestigkeitsniveau R_m = 450 bis 400 N/mm²
• b) Kaltwalzgrad 30 % = R_P 0,2 = 180 N/mm² und R_m = 320 N/mm²
• c) Kaltwalzgrad 50 bis 80 % = R_P 0,2 = 250 bis 280 N/mm² und R_m = 360 bis 370 N/mm²

Example: Steel B

• a) Cold rolling degree 10 to 15% = yield point level R _p 0.2 = 400 to 350 N / mm ² tensile strength level R _m = 450 to 400 N / mm ²
• b) Cold rolling degree 30% = R _P 0.2 = 180 N / mm ² and R _m = 320 N / mm ²
• c) Cold rolling degree 50 to 80% = R _P 0.2 = 250 to 280 N / mm ² and R _m = 360 to 370 N / mm ²

This realization enables one component or function adapted Choice of firmness for one and the same component by change the parameters titanium content and degree of cold rolling.

Table 2 shows that achieved according to the invention Grain size in ASTM units; the achievable grain refinement compared to steels without titanium addition the state of the art is considerable and extends up to ASTM 11 the comparative melts a larger grain and an increased Anisotropy.

For A steel C (variants C3 to C5) were experiments with variable Coil temperature Th and annealing throughput Pg performed (Table 3). While Fluctuations in the throughput of the bell annealer from 1.1 to 1.9 t / h both the grain size as well did not adversely affect the flat anisotropy delta r an increase the reel temperatures to 710 degrees Celsius at approximately the same Rolling temperatures a grain coarsening and an increase in plane anisotropy. The reel temperatures should accordingly between 520 and 710, preferably between 450 and 680 degrees Celsius lie.

Tabelle 2

Tabelle 3

Table 1

Table 2

Table 3

In tables 2 and 3:
Tw final roll temperature
Th reel temperature
K grain size according to ASTM
Pg annealing throughput
Δr flat anisotropy

The cold strip produced in this way after chemical cleaning in an oven under protective gas, preferably an induction furnace, to essentially only the melt bath temperature heated. The term "weld pool" is only here in the Used for an example. Other coating methods, such as. Flame spraying, be used. According to the invention it is important to heat the tape only to the extent necessary for the coating necessary is. With warming so to "melt bath temperature" is a heating up the for to understand the coating necessary temperature.

With galvanizing, the surface temperature is of the tape with this warming not more than 420 to 500 degrees Celsius. The tape so heated is immediately, so essentially without cooling, in the bath for hot dip coating introduced.

The hot dip coated tape is after cooling trained to about room temperature, stretch-oriented and chemical passivated or for a subsequent coating is chemically pretreated and dried. The coating can in particular consist of a multilayer structure, for example from sealing, pre-priming and a multi-layer Pre-painting. Here it is crucial that this hot-dip coated according to the invention Steel band through the warming to only the bath temperature of the cold strip used, even after the paint, shows no aging.

The coating system according to the invention has one immediately before the hot-dip bath 4 arranged heating furnace 14 on. In the furnace, the ribbon is heated to substantially melt bath temperature. In a galvanizing plant, the surface temperature of the strip in the furnace is a maximum of 500 ° C. A temperature of 420 to 500 ° C. is preferably set. An embodiment of the coating system is explained with reference to the embodiment shown schematically in the attached figure.

From the reel 1 the tape reaches a tape storage device 2 (Looper) degreasing with drying 3 in the heating furnace 14 , The heating furnace 14 is preferably an induction furnace which telbar before inserting the tape into the weld pool 4 is arranged. In the furnace, the strip is heated from about room temperature (a), for example at a heating rate of 150 to 300 ° C./s, under protective gas to the desired temperature (b) and then leaves the weld pool 4 with approximately the molten bath temperature (c). In a further strip storage unit with a cooling unit, the strip cools down to predetermined temperatures (d, e) and then reaches a skin pass mill 5 , a stretch judge 6 and chemical treatment 7 with dryer 8th , This can be followed by several coating steps 9 . 11 with respective drying 10 . 12 respectively. The finished coated tape is on a reel 13 wound.

Claims (3)

Process for the production of hot-dip galvanized steel strip with the following process steps: - making a good one Steel strip exhibiting cold formability by hot rolling, reeling and then Cold rolling and recrystallizing annealing in the bell annealer - Warming the Steel strip thus produced and cooled to about room temperature in the uncoiled Condition to a temperature between 420 ° C and 500 ° C with a heating rate of 150 ° C up to 300 ° C / s - right away then initiate the heated Steel strip essentially without cooling in about the same Zinc melt pool at temperature - subsequent cooling with a cooling unit and tempering the hot-dip coated tape. The method of claim 1, wherein the steel strip has the following composition having: 0.02 to 0.06% carbon, preferably 0.03 to 0.048% carbon, 0.01 to 0.40% silicon, preferably to 0.06% silicon, 0.10 to 0.80% manganese, 0.005 to 0.08 % Phosphorus, 0.0005 to 0.02% sulfur, 0.002 to 0.009 % Nitrogen, 0.015 to 0.08% aluminum, 0.01 to 0.04 % Titanium, Max. 0.15% of one or more of the elements copper, Vanadium, nickel, remainder iron and unavoidable impurities. Hot-dip galvanized, cold-rolled and age-resistant sheet or steel band by the method according to claim 1 or 2.