DE19543804A1 - Process and plant for hot dip coating of aging-resistant steel strip - Google Patents

Abstract

Producing cold-rolled steel strip comprises heating the steel strip before an immersion-coating in a protective gas, to a temperature below the recrystallisation temperature to not more than 420-500 degrees C, in which the steel strip comprises (%): 0.02-0.06 C, 0.01-0.40 Si, 0.10-0.80 Mn, 0.005-0.08 P, 0.0005-0.02 S, 0.002-0.009 N, 0.015-0.08 Al, 0.01-0.04 Ti, maximum 0.15 Cu, V or Ni. Also claimed is an apparatus for carrying cut melt immersion coating comprising a furnace (14) arranged before the melt immersion bath (4), in which the temperature of the steel strip is adjusted to a value below the recrystallisation temperature to the temperature of the bath. Further claimed is the steel strip produced, consisting of the above components.

Description

The invention relates to a method and a plant for hot dipping layering of steel strip. The invention also relates to a Ver driving available hot-dip coating, in particular galvanized Steel band. This steel tape is said to be resistant to aging, a good one Have cold formability or deep-drawing ability, in particular be particularly suitable for producing body panels. The steel band should also have a quasi isotropic deformation behavior (small delta r value) and have a high yield strength.

According to DE-OS 15 21 376 it is known that the tapes before the introduction to that Heat zinc bath to 450 to 500 degrees after reducing in beforehand the atmosphere the tape at a temperature of 720 to 950 degrees has been glowed.

It was also known from the same script to use annealed strips add, where no annealing treatment takes place. Instead, the Tapes heated up to 230 degrees Celsius and with these surfaces temperature through the zinc bath heated to 450 degrees Celsius. The However, this method has the disadvantage that the zinc bath is hosted by an unwanted Community induction heating must be heated from the outside and this Difficulties arise. It was therefore suggested the tapes continue to soft anneal, clean and open in a reducing atmosphere to heat a temperature between 500 and 720 degrees Celsius before the strip is inserted into the zinc bath at around 480 degrees Celsius. Here should achieve a yield strength of only 20 kg / mm².

The annealing treatment at temperatures above 500 degrees Celsius The introduction of the tape into the zinc bath is also well known State of the art provided as a basic measure.  

For example, in DE-OS 16 21 340, the tape at temperatures between annealed between 650 and 750 degrees.

According to DE-OS 23 40 413 the tape is under non-oxidizing conditions s heated to 750 to 800 degrees Celsius before being introduced into the zinc bath becomes.

According to DE 27 11 041, the strip is heated to 650 to 950 degrees Celsius, before immersing it in the zinc bath after cooling.

According to DE 40 38 186 C1 in the hot dip process surface ver noble tapes can only be produced in a continuous process. Here after an annealing process, the usual aging treatment, where the tape is below 450 degrees Celsius for at least 60 seconds must be kept by cooling in three cooling phases with each defined cooling rate. This is intended to achieve that the yield strength of the steels treated according to the invention is reduced and the elongation at break is increased. How to take samples after a Outsourcing of four weeks were checked in order to achieve normal storage times between to take into account the manufacturing and processing, oi ne improvement in aging resistance has been achieved. This favorable effect is achieved by cooling with the invention decrease in the harmful, dissolved carbon content call. In this case too, the recommended cooling goes with defined Cooling speeds an annealing treatment from 750 to 800 degrees Celsius ahead. It also shows that it is not an aging resistant sheet with a high yield point.

The object of the invention was therefore to provide a hot-dip coated steel provide tape that is resistant to aging. Another task is Manufacture of a hot-dip galvanized sheet or strip that is high Yield strength or high strength with a good Cold formability or deep drawing capability combined and a quasi has isotropic pulling behavior.  

A solution to this problem, hot-dip coated cold strip is obtained by adding the cold strip, which, for. B. has the following analysis:
0.02 to 0.06% carbon, preferably 0.03 to 0.48% 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 elements copper, vanadium, nickel,
Rest of iron and unavoidable impurities,
is recrystallizing annealed in the hood furnace.

The cold strip is then essentially before immersion in the weld pool only warmed to molten bath temperature. In particular, re the temperature of the strip surface when galvanizing to no more than 500 degrees Celsius increased. The tape appears immediately after this heating into the weld pool. The advantage here is that for example probably with the soft unalloyed grades as well as with the higher-strength micro alloyed grades, the coated sheet is independent of the coating type of treatment (for example electrolytic galvanizing or hot-dip galvanizing) same chemical composition and the same technological egg have properties. The technological properties of the coating steel straps are not only of the chemical composition of the Steel depends, but also on the heat treatment that the be layered sheet steel has passed through. For strip galvanizing, the Technology basically moves in the direction with strip galvanizing to combine a continuous annealing. The tape will be on the above written high temperatures heated. To use this procedure the to produce the same technological properties as, for example in the case of annealed material (e.g. ST14) in conjunction with a Electrolytic coating must be achieved for hot dip galvanizing a higher quality material (e.g. IF steel) can be used. The  now proposed, in connection with the previously used continuous annealing with a hot-dip coating leads to different procedures to the essential advantage that the processing of the coated ble ches no longer has to be adapted to the type of coating. That's how they are same technological properties with the same chemical analysis, achievable regardless of the type of coating.

In addition, the hot-dip galvanized steel strip according to the invention is without additional measures - such as defined cooling phases - old resistant to aging.

Finally, since the continuous annealing used to date Recrystallization is no longer necessary, the coating treatment no longer match the continuous annealing. When galvanizing is this no longer depends on the recrystallization treatment in the Continuous annealing. Therefore, the belt speed is no longer on the To coordinate steel grade. The performance of the galvanizing system is consequently only determined by the design of the galvanizing part.

The hot-dip coated steel strip according to the invention is first regarding the cold strip production based on the following example games for the production of a cold strip with extremely low anisotropy (Delta r <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 heating When the pusher was heated to 1250 degrees Celsius, the slab became hot strip 3 mm thick rolled, coiled and cooled to room temperature. The roll end temperatures and reel temperatures are shown in Table 2 The strips were pickled by cold rolling in different stages from 10% to 80% reduced to thin sheet thickness and re-coiled. The bundle was heated in the bell annealing furnace to 700 degrees Celsius, with egg Throughput from 1.1 t / h to 1.9 t / h recrystallizing annealed and then cooled in the oven to 120 degrees Celsius.

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 alloying element titanium, for example 0.025% Ti, cold rolling grades of up to 15% or 20% and up to 85% are preferred posed. Values between 0.01% and 0.02% are preferably low Lower cold rolling degrees set. The preferred cold rolling grade could be one high tensile strength and yield strength levels can be assigned.

Example: Steel B

• a) Cold rolling degree 10 to 15% = yield point level R _p 0.2 = 400 to 350 N / mm²
  Tensile strength level R _m = 450 to 400 N / mm²
• b) Degree of cold rolling 30% = R _p 0.2 = 180 N / mm² and R _m = 320 N / mm²
• c) Cold rolling degree 50 to 80% = R _p 0.2 = 250 to 280 N / mm² and R _m = 360 to 370 N / mm².

This knowledge enables a component or function-adapted choice of Strength for one and the same component by changing the parameters titanium content and degree of cold rolling.

Table 2 shows the grain size achieved according to the invention 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. In contrast  the comparative melts showed a larger grain and an increased ani sotropy.

For a steel C (variants C3 to C5) tests with variable has temperature and annealing throughput Pg carried out (Table 3). While Fluctuations in the throughput of the bell annealer from 1.1 to 1.9 t / h both the grain size and the flat anisotropy Delta r not had an increase in reel temperatures to 710 Degrees Celsius at approximately the same roll end temperatures coarsening and an increase in the level anisotropy. The reel temperatures should therefore be between 520 and 710, preferably between between 450 and 680 degrees Celsius.  

Table 1

Melt analysis (values in percent by weight)

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 is after a chemical cleaning in an oven under protective gas, preferably an induction oven, on the we only heated the molten bath temperature. The label "Melting bath" is used here only in the sense of an example. It can other coating processes, such as. B. flame spraying used will. According to the invention, it is important only to extend the belt as far warm as necessary for the coating. While warming up "Melting bath temperature" is therefore a warming up to that for the coating to understand necessary temperature.

With galvanizing, the surface temperature of the strip is This warming does not exceed 420 to 500 degrees Celsius. That so warm The tape is placed in the bath immediately, essentially without cooling introduced for hot dip coating.

The hot dip coated tape is cooled to about Dressed at room temperature, stretch-oriented and chemically passivated pretreated chemically for a subsequent coating and dried. The coating can in particular consist of a multilayer construction, for example from sealing, pre-priming and one more layered pre-coating. This is of crucial importance tion that the hot-dip coated steel strip according to the invention by heating to the melt bath temperature of the cold used bandes, even after painting, shows no aging.  

The coating system according to the invention has a heating furnace 14 arranged directly in front of the hot-dip bath 4 . In the oven, 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 degrees Celsius. A temperature of 420 to 500 degrees Celsius is preferably set. This eliminates the usual cooling and aging. An embodiment of the coating system Be explained with reference to the embodiment schematically shown in the accompanying figure.

From the reel 1 , the strip reaches the heating furnace 14 via a strip store 2 (looper) via degreasing with drying 3 . He heating furnace 14 is preferably an induction furnace, which is arranged immediately before the introduction of the tape into the molten bath 4 . In the furnace, the band is heated from about room temperature (a), for example at a heating rate of 150 to 300 degrees Celsius / second under protective gas, to the desired temperature (b) and then leaves the melt pool 4 at about the melt pool temperature (c). In a further strip store with a cooling unit, the strip cools down to predetermined temperatures (d, e), then passes into a skin pass mill 5 , a stretching judge 6 and a chemical treatment 7 with a dryer 8 . This can be followed by a number of coating steps 9 , 11 , each with drying 10 , 12 . The finished coated tape is wound on a reel 13 .

Claims (8)

1. A process for the production of hot-dip coated cold-rolled steel strip, characterized in that before a dip coating, the steel strip is heated under protective gas to a temperature below the recrystallization temperature, essentially only to the molten bath temperature, the steel strip, in particular, having the following composition in percent by weight:
0.02 to 0.06% carbon, preferably 0.03 to 0.48% 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 elements copper, vanadium, nickel,
Balance iron and unavoidable impurities. 2. The method according to claim 1, characterized in that with galvanizing the strip to no more than 500 degrees Celsius, in particular heated to 420 to 500 degrees Celsius. 3. hot dip coating system, characterized in that a heating furnace is arranged immediately in front of the hot-dip bath net is in which the temperature of the steel strip to a value below half of the recrystallization temperature, essentially only to the tem temperature of the weld pool is set. 4. Coating plant according to claim 3, characterized in that the temperature in the heating furnace for galvanizing is a maximum of 500 Degrees Celsius, in particular 420 to 500 degrees Celsius.   5. Hot dip coated steel strip, available from Haubenglü Hung the cold strip and warm the strip before diving stratification to a temperature which is the melting bath temperature not significantly exceeds, especially not greater than 500 degrees Celsius is. 6. Hot-dip coated, cold-rolled steel sheet or strip with the following composition in percent by weight:
0.02 to 0.06% carbon, preferably 0.03 to 0.048% carbon,
0.01 to 0.40% silicon,
0.10 to 0.80% manganese,
0.005 to 0.08% phosphorus,
0.005 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 elements copper, vanadium, nickel,
Rest of iron and unavoidable impurities,
obtainable by heating the recrystallized annealed strip to essentially only melt bath temperature, in particular to not more than 420 to 500 degrees Celsius. 7. Hot-dip coated, cold-rolled steel sheet or strip with the following composition in percent by weight:
0.02 to 0.06% carbon, preferably 0.03 to 0.048% carbon
0.01 to 0.40% silicon,
0.10 to 0.80% manganese,
0.005 to 0.08% phosphorus,
0.005 to 0.02% sulfur,
Max. 0.009% nitrogen
0.015 to 0.08% aluminum,
0.01 to 0.04% titanium,
Max. 0.15% of one or more elements copper, vanadium, nickel,
Rest of iron and unavoidable impurities,
obtainable by coiling the hot strip at less than 700 degrees Celsius and then cold rolling and annealing, the cold rolling taking place depending on the titanium content and by heating the strip to essentially only melt bath temperature, in particular to not more than 420 to 500 degrees Celsius. 8. Use of a hot-dip coated cold-rolled sheet or steel strip according to claims 1 to 7 for deep drawing of automotive parts.